There really aren't any.
Your best bet is to breed something that's easy to breed and is popular, but not so popular (like cons) that you can't sell them. There's probably not a lot of demand for expensive fish that grow to large sizes. Rays aren't in large demand because the average fish keeper doesn't want to buy expensive fish that they don't even have the room to house. You want something that's low maintenance and that is affordable for the average person. I think most ray breeders do it out of sheer love for rays and not to profit.
I'd ask your LFS what is their best selling fish and go from there.