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As far as sources for doors go, you will probably have to find a used one. There are a few used RV parts dealers that have Airstream parts. Silvertrailersupply.com is one of them. You might also check the "resources" section of vintageairstream.com, there are a lot of good parts resources compiled there. (Restoration Suppliers & Resources - Vintage Airstream
Now then, when you see how hard it is to find a complete, and in good condition door (and the cost of the door plus shipping), you will be inspired to exhaust every effort to repair the one you have. Search the Forums for threads describing "sprung doors." There are a few threads on this topic that explain the whole process of straightening out a door. First you will need to find someone locally who can do TIG welding to repair your cracked frame. This is potentially a large, and involved project depending on how far you are willing to go. As long as you are in there, you might consider replacing the old KT latch system with someing more modern (which will also require modifications to your door frame and striker pocket.