I googled a bit and found some 30mil film:
http://www.xpel.com/products/bulk.asp
bottom of the page.
There is a data sheet link (in red) that describes the product.
I saw some eBay products in the google results, but didn't check them out.
When I run out of what I have, I'll try the 30mil X-PEL product. It's under $20 for a square foot.