Fauna classifieds usually has a lot of various critters for sale. There is even a trader rating for each poster that you can see on each sellers profile page, although not a conclusive way to judge reputation because some people don't leave feedback either positive or negative after a purchase. There's also a subforum called the BOI on Fauna where you can inquire about a seller's reputation. Always do a little bit of checking + asking questions before purchasing over the internet.