Isopods are detritivores. They eat dead and decaying organic material - mostly plant matter. Some isopod species need or prefer more protein (i.e., animal products) than others. When you're culturing isopods in captivity, they can eat:
Substrate
Compost, vermicompost, leaf litter, decaying wood, bark
Commercially-made isopod diets
Fish flakes, dog food, or roach chow
Freeze-dried shrimp, minnows, or krill
Fresh vegetables like potatoes, carrots, and squash
A source of calcium is also crucial since isopods are technically crustaceans. Cuttlebone, eggshell, and crushed oyster shells are great additions to their diet.