Walmart 2.2lb for 5$ and each fillet is air sealed. Last me a while I cut up 6 fillets into some cubes so my Ray can eat and put it into a zip lock bag for easy access. Every feeding I just use a small container oh tank water and let it defrost for a few minutes and feed it. When I first cut it up I made the cubes too big and it was hard for the Rays to eat they would have to break it and eat it.
Great to see your rays acclimating well. They're beautiful! Great tank also. Your rays will devour the black worms. Also try some redworms/red wigglers until you can train them off live foods. Best of luck!