I work for a bank and we get very large bound reports in, usually in triplicate. We keep one, send one to the customer, and the third one is usually trashed. Some of the covers are thin and clear and I grab those for my cards. The other covers are very thick or frosted (but you can still see through them) and I cut those down and use them with my stamp a majig.
Another item I found was a non stick sheet that goes in the bottom of your oven for about $5. I grabbed one of those and they work great! Just like the expensive Tim Holtz mats!