I use chicken food, DuMor Layer pellets. I like the pellets because you can sift out the poop and keep the remaining food. Also less is more, I recommend only using a thin layer of whatever you decide on because you can always add more food.
I use wheat bran and oats because I can buy it local.However I find it more expensive so eventually Im going to switch to the chicken layer pellets(its higher in calcium then bran too).