When I first got into this hobby about ten years ago it was very popular in Europe but not here in the US yet.I was looking for an alternative to a fish tank and came across Vivaria's site. So I purchased my viv from Vivaria in Holland which wasn't cheap. But now there are places that offer setups in several different sizes locally for much less or you can design your own and save money.
Yes they are legal and there are many breeders all over the US and Europe as the hobby has become more popular over the last several years. They are poisonous in the wild due to their diet in the rainforest. When captive bred and offered a diet of fruitflies or something similar they don't secrete poison like they are known for in the wild. Local natives have known about this for centuries and dip their arrows/darts in the poison for hunting so that is how they got their name.
Anyone interested in this hobby can go here for more info on raising these beautiful creatures.