They get a "list" once a week or once a month from whichever suppliers/breeders they order fish from. If it's not on the list they can't order it.
If you are thinking about the brevis or occies, I would definitely recommend the occies, they are very entertaining. The brevis are slightly easier to breed, but are a little boring otherwise. I'm thinking about selling my brevis to make room for another species, but I won't be giving up my occies anytime soon.