Your best solution will probably be to use live food. Several foods are easy to keep, and you can even sell cultures to other hobbyists once they're thriving... Yes, you can make profit selling them, and get free fish food.
Some of the easiest to grow (and most profitable) foods are
daphnia (feed green-water, or a mixture of tea made from spirulina powder and egg yolk)
White worms/Grinal worms (feed tiny amounts of oatmeal)
Blackworms (can be raised in a fishless aquarium easily, will eat fish food..)
flightless fruit flies
Some information can be found here- The Bug Farm - your source for live food culture information.
As for the blackworms, here's a link-
Black Worms: Keeping and Breeding
But, I advise a more natural approach..Start with a rubbermaid container or small aquarium.Add a tiny bit of soil (round tablespoon) and a cup of water to about an inch of gravel from an established aquarium. Cover with clean gravel (from your aquarium)l- Fill with aquarium water to a depth of 6 inches. Add some plants for filtration (or a sponge filter), and you shouldn't ever have to restart the culture as long as you give them water changes when you do your fish.