Nemss316,
You can goto a craft store and get foam board any colour and just cut it to what you need. It will cost between $2-$10 depending on thickness. Most foam board will have a white back, white foam inside and colour front. The colour front usually comes in a matte finish. If you can't have any white showing on the inside edges, there are black ones all the way through as well. Don't use marker to colour the inside foam because it will shrivel up the foam. (You can form the foam and actually bevel the edges quite nicely with practice using a magic marker). This reply is only for a do it yourself solution.
Herk