I don't necessarily breed worms for the fish. I do, however, maintain a large compost pile which has attracted thousands of worms. I started the pile more for the fishing bait aspect than for fish feeding.
The worms live at the bottom level of old compost. New compost is turned in at the upper layers so the pile doesn't generate too much heat for the worms. They regulate how far into the new material they want to go for a fresh meal of fruit and veggie throw-aways.
As the pile builds up, I remove bottom soil (full of worm castings) to thresh into our raised gardens and for repotting our house plants.