I assume you mean the "Tambour" door made up of a bunch of little segments, and it rolls to the side? Have a look at the Airforums Classifieds. There was a guy trying to give away the entire interior of a 70's era trailer, he might have a serviceable door among his stuff. If you can't find a used one, you will be in a bit of a bind. You can buy "sheets" of the tambour door material, but it is really expensive. I believe Inland sells it. You may also be able to repair the tambour by gluing the existing "door to a piece of canvas, or similar material.