I use mostly oatmeal. Sometimes I throw in a bit of bran if I have it, but they like the oatmeal just fine. They also like a piece of potato now and then, but be sure the cut potato sits on the counter for 24 hours or so otherwise it may mold against the oats. The potatoes leak fluid for a bit when cut.
i go to the bigger supermarkets and i get the bulk organic 10 grain mix. its all crumbled up so they can munch it and i add veggies on top. its cheap too($1-1.50/lb). i use about a two inch thick bedding and buy 1000 supers at a time. they are mostly for beardy and geckos tho. lucky loves his crickets.