Jar lids/bottle caps. I use the ones from 2 gallon cat litter jugs(I go through a lot of litter) and they work great. After about a month they graduate to adult dishes, which are the ringed disposable plastic kind from The Bean Farm.
We use Vitamin Water lids. We've also cut the tops off of small styrofoam cups. Not very eco friendly but if you don't have any juice bottle lids it works pretty well.
I use the small solo cups, they can tip them over but I put them next to the hide and most will just perch up top and eat right from the cup. The ones that tip and make a mess I put the solo cup into a 2" pvc end cap from the depot. They can't knock those over.