Sharks and sting rays are related, and there are freshwater rays. I've never heard of freshwater sharks, but the saltwater kind, as we all know, are quite dangerous. Aquarium shops sometimes sell what they call red tail sharks, but I doubt they're related to the real thing. Do the aquarium ones have teeth? Maybe if the sales person could give you their Latin name, you could Google it.
As a a general rule, if you rarely see a certain species in an aquarium, it's because they're difficult to keep. I think some people get out of the hobby, because they start out buying some pretty exotic, expensive fish only to have them die.
