Acceptable foods for discus are:

Dry Foods (always go for premium as some cheaper brands are no more than cereal)
Flake foods (we recommend Tetra)
Pellets (we recommend Aquatic Revolution, Spectrum ,Tetra Colour Bits)

Frozen Foods (always go for premium )
Blood worms (we recommend Orca)
Mysis shrimp (we recommend Posaqua)
Brine shrimp (we recommend Posaqua)
Beef heart mixes (we recommend Posaqua Discus Diet)

Live Foods
Black Worms
Brine Shrimp
Mosquito Larvae

When feeding live foods always rinse well as there can be a lot of dead rotten worms in the pack you get.  You will need to rinse the salt water from the live brine shrimp or you will change the chemistry of your water.  Also make sure you worm your fish regularly.

You can also make your own foods from fresh produce.