Foamies are sheets of relatively flat foam. I've seen it come in thicknesses from 1mm up to 6mm. You can get them at pretty much any craft store, and also at Wal-Mart.
Here is a page with some of them:
I don't mind. ^.^ I'd love to see others put these efforts to work...its why I post them! Just be sure to give proper credit if you're asked.