Mounting foam for unmounting stamps

A question came up in the comments on my unmounted storage about where to get the mounting foam. I get mine from Beeswax Rubber Stamps. Their price is the best I've found, and it works great. You can get quite a lot of stamps mounted on a single sheet of it. If you're not indexing the back of the foam, you can really cram them on there with just enough room to cut around. Since I opt to stamp the index on the foam, I'm not quite as economical in my usage, but the trade-off was worth it for me. I like being able to see the image through the block, and also so see the image through the CD case without having to squint at the reverse image on the rubber, especially on text stamps.