Try aquabid? Some people will buy them in bulk, possibly to resell them at a pet store. If you breed some other fish, some people there will sell the other fish and add in the cories as an optional bonus, or say you can only buy a minimum of 5 at a time.
I think cories are a good beginner fish to try to breed. Idk, I've never done it, but I've always wanted to. Cories have got to be one of my favorite fish. They remind me of a pack of tireless, hyperactive dogs lol!