If you are looking for something with which to replace the door panels, which is what this thread sounds like, use tempered Masonite. Available from any building supply. I used it in my 450 slc and it worked great. Eighth inch plywood works better but it is sometimes hard to find. The little plastic studs that hold the panal to the door are available at MB or at your local parts shop.

You can also staple the covering to Masonite which is easier than glue.