I buy earthworms for my ray from a local fishing bait shop.
I have however started off trying to breed them in the garden.
I made a 2 foot square box and filled with peat moss soil and compost leaf litter.
I added a box of earth worms.They should start reproduction within 1 month.
I just keep it damp and add a little green waste now and then.
By the way, my rivulatus lived earthworms.
All compost must be organic with no added chemicals.