Species of geophagus, red crystal shrimp, or most species of pleco that are un-common would do well for selling fry i think.
Stingrays do fetch a high profit per stingray, but with the extremely large tanks you would need, and I mean thousands of gallons, and the low birth numbers compared to most fish, I feel that it would break even in comparison to some other fish. You could breed millions of fish in the same space and costs as maybe a hundred or so stingrays. With all that being said though. If I had the money I would totally invest in breeding stingrays, just because of how cool it would be.