Check out your local newspapers. They can sometimes give away free unprinted roll ends. It's typically good quality and clean.

Here in Ottawa, I go to the local paper once in a while and am able to get roll ends without a problem and free of charge. They're usually thrilled to have it off their hands. They have a max of 3 rolls per person and, since I only have one bp for now, those 3 last an eternity.

Hope this helps.... I know that saving a bit of cash on the substrate helps me put that much more towards new snakes or enclosures.