Well starve them for a few days. Then maybe try market shrimp or what i have had better luck with is tilapia (frozen bags from walmart). Can get it in 2lb bags or 4lb bags. Id get a 2lbs bag first adn see if you can get them to eat it. Starve them for a few days, maybe even three and then cut your tilapia in small pieces (bit size to start). See if you can get them on that. Will help with costs.
Blackworms are great, but can get costly. I used canadian night crawlers and it worked well also (they are smaller than earth worms) (cut in half if you have to) but that also can get costly.
Other ways is to get a small tank and buy some guppies. Put in some plants and after awhile, they will breed like crazy. Good way to raise live healthy feeders for the rays to play, chase and eat from time to time.