Here are my views on the responsibilities and hazards of keeping tarantulas, along with some options in purchasing them.
First, keeping a tarantula is a serious responsibility. If you buy an adult, you are getting an animal that is 3 - 10 years old and deserves to be treated with respect. They will live for many years, if well cared for. Tarantulas are easy to care for and need relatively little attention, but need it reliably throughout their long lives. Tarantulas can not be released outside when you are tired of them. (With the internet, it is now easy to sell it to someone who is interested or consider donating her to the local Nature Center.) Most tarantulas come from tropical areas, so need to be kept relatively warm and not left in chilly areas of your house. You would not want to carry one outside in the winter, even briefly, without wrapping the cage well. Surprisingly, because of their large size, tarantulas tend to be relatively delicate. The exoskeleton of the abdomen is relatively thin. If dropped, they tend to splat and bleed to death. For the spider's sake, I strongly suggest not handling tarantulas. They are elegant, interesting animals, but not cuddly pets. Just as you would not expect to pet your fish, do not get a tarantula with the goal of handling it.
Care for tarantulas is relatively easy: Feed them once or twice a week depending on their hunger levels, keep humidity high in the cage (if a tropical species), and make sure that there are no crickets in the cage when the spider moults or else the cricket will kill the spider. Crickets or flies are better than mealworms or superworms. Don't let their cages dry out. Give them water. Your spiders do not need large cages and smaller animals will do better in smaller containers than larger ones. Keep them on a sterile substrate soil (potting soil without minerals added, vermiculite, commercial top soil, pet bark [not cedar], peat - NOT soil from your yard!) which can hold moisture. Provide retreats and some environmental enrichment (bark to climb on, something to hide under). Plants are for your aesthetics, not for the spiders. No sharp cactuses or other plants that can injure the spider.
First, tarantulas are wild animals, not long domesticated pets. For example, I have 10 year old tarantulas that I have reared since they were in the egg sac. They have always been in captivity in cages in my lab. I have always been gentle, slow moving, and treated them with care. But the species is active, aggressive, and fast. I would never even consider attempting to handle one - they have 3/4" fangs and a feisty nature! If I have to move my spiders to a new cage, I gently push them with long forceps, have capture cups nearby, and I am always cautious. Even calm species that are considered 'handleable' can get aggressive if they feel threatened. As an aside, I always transfer my spiders with the cage on the floor. That way, if they do make a run for it the tarantula is on a flat surface and it is easy to get a cup over it. I don't want to admit how many times I've lost spiders that climbed into desk drawers, had them fall off tables, or had to dig them out of boxes. I know other people who do their transfers in their bathtubs (drain covered), again to control where the spider can go.
Second, although most tarantula species appear not to be poisonous to humans, there is always the possibility of a person's individual reaction to a bite. Tarantulas only have venom glands in their chelicerae, as opposed to glands that extend into their cephalothorax. The bite of most species is described as being similar to a bee sting. The more important damage is likely to come from mechanical damage from a bite. Their fangs are easily strong enough to break skin and penetrate some depth into your flesh! Why risk it? Do not put your spider in a situation in which it feels it needs to defend itself by biting. Minimize contact and bites are unlikely to occur. There are a few species which have turned out to be both relatively venomous and fast moving [Ornamentals (Poecilotheria sp.), Starburst baboons (Pterinochilus sp.), & Cobalt blue (Haplopelma sp.)]. I recommend that the first couple of tarantulas, particularly for classrooms, be relatively calm, non-aggressive, and not zippy species.
Third, tarantulas from the New World have developed another very effective defensive behavior. When threatened, they rapidly scrape special hairs, called urticating hairs, from the back and sides of their abdomens into the eyes, nose, or lungs of their vertebrate predators. The hairs have a variety of shapes and are coated with irritating chemicals that cause itchy skin or irritated membranes. I have known of several people who have gotten the urticating hairs of tarantulas in their eyes and have needed surgery to have them removed. This is the most serious risk that you are likely to experience with your spiders. In all honesty, I've rarely experienced the urticating hairs except when handling shed moult skins. A stressed animal is likely to flick them at you with her hind legs. Wash your hands after handling moult skins, do not open a cage and look at your spider directly. Some species are more likely than others to flick hairs.
The other hazard of having tarantulas, instead of araneomorph spiders (like jumping spiders or orb weavers), is that they cost real money and there is no guarantee that they will survive. Recognize that your animal may not survive for a number of reasons: normal mortality while moulting, some spiders never thrive (especially spiderlings), your learning curve, cricket predation, or roommates who leave cages open... With care and good fortune, your tarantula will live for years, but it does not always happen that way.
You can buy a perfectly fine tarantula at one of the local pet stores. I recommend trying to determine if it is a male in the store. Look for enlarged pedipalps and the tarsal spur. Once males reach maturity, they cruise for females for 1 year and then die. If it is a male, you will have an active animal for the year he has left to live, but he will certainly die relatively soon. Many of the tarantulas in pet store are males. Prices of spiders in pet stores are comparable or at the high end of spiders from mail order suppliers. More critically, the tarantula species in pet stores are often very limited and the store employees may not know how to advise you in the care of your species.
An alternative is to order a tarantula from an Internet tarantula dealer. The downside of buying tarantulas through the Internet is that you don't get to pick the exact individual you want, you need to pay for shipping (check ahead of time, it should be $20 for overnight delivery), and you need to trust the dealer. There are a couple of reasons why this might be a good alternative for you: 1) It is possible to get much more interesting species this way, 2) you can get younger animals who will live longer, moult more frequently, are somewhat more active, and usually captive born, and 3) you will not get an adult male. Shop around the Internet to find a good, reputable dealer who will work with you on choosing the appropriate species.
Some species are more active and generally interesting than others, some are calmer, some are interesting architects, others are flashy predators, some are just out and out lovely, but others are just plain boring. So, like the breed of dog, the tarantula species also matters. To choose just the right animal for your needs, try to come up with the right balance between your patience, squeamishness, and budget. If you get scared by fast moving spiders, pick a slower moving species. If you really want to pet your spider occasionally and not worry about it biting you, get a pinktoe. First, think about what age to purchase:
Spiderlings: I've loved raising spiderlings, rather than buying adults. In all honesty, almost my entire collection was started with inexpensive spiderlings because I couldn't afford adults. Captive reared spiderlings are somewhat more active than adults, I've enjoyed seeing developmental changes in their anatomy and behavior as they mature, and it is fun to see the huge changes as a small animal moults regularly into a large animal. I have personally found them more gratifying. Most of the beautiful animals I have were 1" in size when I bought them and they are now good sized animals. However, smaller spiders are often more delicate, more likely to die while moulting, are harder to feed (where are you going to get small flies or crickets in the winter?), and can be much shyer than the adults (or more active). If you've got patience, the spiderlings can be an inexpensive way to learn about tarantulas. Look at the body size on a price list and don't choose a spiderling that is less than 1" in size. A 1" spider means body and legs combined to equal an inch. It is easy for that size to be exaggerated on price lists. Depending on the size of your spider, you would start by keeping them in a big pill vial, and eventually move them to larger cages. Remember that tarantulas have terrible vision and will have trouble finding the prey in large cages. Small spiders NEED to start in tiny cages, then move them to larger spaces as they grow larger.
Adults: If you are new to keeping invertebrates, you might want to start with a big, super easy adult or subadult. Adults are easier to feed, do not need quite as regular feeding, have survived to adulthood, and are bigger and easier to see. Adults are fine animals to have. They are also hardier and more likely to survive in a classroom. Adults (that you can afford) are generally wild caught, while the spiderlings are generally captive born. All of the species I am recommending are both common and commonly bred in captivity. Like the spiderlings, all tarantulas need a retreat in their cage to hide under. Adult spider probably prefer to have more space in which to move. You can keep them happily in small cages to 10-gallon tanks -- just make sure that it has a secure top on it! I generally cover most of the tops of cages with plastic wrap or tape because the spider can dry out too much with a totally open screened top to its cage. In all of my cages, I have soldered or drilled cork-sized holes so that I can put crickets in the cage without the need to OPEN the top of the cage. With many species it is easy to 'hand-feed' crickets to the spiders through the holes without direct contact between you or your spider.
Think about the life style of your spider. Decide between an arboreal or a terrestrial species. Arboreals, or tree living species, not surprisingly like to go up and stand on the side of the cage as they would on a tree. In practical terms that means you need to provide a taller cage that allows them space to move up and down on the side of the cage, but doesn't need a big 'footprint'. Some arboreal species produce more silk (which can block your view), but in my experience tend to be more active. Other than the Pinktoes, they are often faster moving species. I happen to love the long legs and sleek, elongate bodies of the arboreal species and find the fatter terrestrial species to be rather clunky.
Terrestrials spend most of their time sitting on the ground. Some dig burrows which make them harder to see, but most sit in the open. Some of them are very slow moving, although some are faster. Not surprisingly, terrestrial species need more area on the ground but not much height in their cages. They need a retreat of some sort.
Any Pinktoe (Avicularia) species - Very gentle, arboreal. A. urticans, A. avicularia, A. aurantica are all lovely, hardy, calm species. They are ideal first spiders. The only problem I've seen is that they get very inactive prior to moults, and premoulting behavior is not as obvious to a first time owner as with some of the others. Although they are very calm spiders and generally slow moving spiders, they are capable of making a fast run for it if you are careless They are not very active, but nice, calm spiders.
Costa Rican Zebra (Aphonopelma seemanni) - Terrestrial, fairly active, attractive, inexpensive, and not particularly aggressive. Again a popular first spider.
Honduran curly hair (Brachypelma albopilosum) - Terrestrial, fairly active, inexpensive, and not particularly aggressive. Moderately rapidly growing. Again a popular first spider.
Mexican redknee/leg/ red rumps (Brachypelma smithi, B. emilia, B. vagans) - Terrestrial. Any of the Brachypelma species are great first spiders. Behaviorally, they tend to be calm and relatively inactive, but they are so lovely it compensates for a more calm spider. The redknee is beautiful, and grows to a large size, but is protected because it has been so popular with collectors. The Mexican redknee (Brachypelma smithi) is calm, lovely, slow moving, amazingly slow growing, and expensive. The Mexican red rump (B. vagans) is common, moderately fast growing and moderately active with a pretty red rump. The red leg (B. emilia) or flame leg (B. boehemi) are lovely animals.
Trinidad Chevron (Psalmopoeus cambridgei) - Arboreal, fast growing, really active, large, & interative. One of my favorites because they have interesting and lively behavior, but this species is very fast and is aggressive. I like to give them crickets directly through the cork hole. This is only a species to get if you feel calm about dealing with a fast spider that will eventually run up your arm. This is a species that I breed often. The last time we transferred 1" spiderlings into small cages, at least 5 ran up our arms into our clothing, biting as they ran! Not a species for the timid. They need to be in cages that allow them to climb on the side of the cage and stretch out. They are not spiders for beginners, but they are one of my favorites.
African Baboons (Hysterocrates sp.) - These are burrowers that spend an incredible amount of time moving soil around the cage, serious architects with interesting behavior. Need to be kept very moist. Terrestrial, fast growing, very active, complex burrows that are generally easy to see into. Can be kept in social groups for the first year of life. Another one of my favorites. Not aggressive until adults, but is reported to have a somewhat venomous bite.
Many other species are interesting and beautiful: Orange blue bottle (Chromatopelma cyanopubescens) - awesome colors, active, interesting web, pricy; Acanthoscurria geniculata - lovely, active, interesting. My passion are the Ornamental tarantulas (Poecilotheria species). They are gorgeous with lovely patterns, sleek, huge, and fast. But they are venomous and are absolutely not the tarantulas for beginners.
One species to avoid: the Chilean Rose Hair (Phrixotrichus spatulata, often called Grammostola), which is the most common spider in pet stores. Chilean rose hairs are very gentle, slow-moving, lovely, cheap, but a painfully boring tarantula. I am not convinced that this tarantula has any behavior in its repertoire. I've never seen such boring animals, they are hesitant about attacking prey and just don't move much! Buy a rock instead. I recognize that some people love their rose hairs and find them interesting animals. I don't find them at all interesting. Please don't write to tell me about yours!
Other Arachnids: Emperor scorpions - inexpensive, adults, can live in groups, non-venomous. I personally don't find scorpions as charming as spiders, but if scorpions appeal to you, this is the species to purchase. Consider getting a male & female as they live in social groups. Also, plan on getting a pair of long forceps for general control.
Consider purchasing long forceps. (Nice to pull crickets out of cages & move recalcitrant spiders.)