You can try copper flashing as an alternative and you should be able to find it at your average hardware store.

Unfortunately, a good hot rodding company, RodDoors, has gone out of business, but their website is still up with pictures on how to replace your doors. [url]http://www.roddoors.com/pages/f-t-f-intro.html[/url] If you're looking to save some money, you can use a heavy cardboard, plywood, masonite or thin aluminum.