Hi retrvrlabs, a big welcome to Air Forums. I see you have a 73 Ambassador. Airstream redesigned the body in 1969 and stayed with it until the "wide body" modification in the 90s. There are lots of used compartment doors out there that will fit.
Get the height and width of the opening. Some of these 70s trailers had a battery compartment door below the joint between the side skins and the wraps to the belly pan of the trailer. Even take a picture of the opening so others will know exactly what you need.
Note the Air Forums TAB at the top of the Air Forums called Classifieds. You can search here, and you could post "Compartment Door Wanted" ad on the Classifieds along with the dimensions of photo of the opening. Someone may be able to help you.
Link: Parts and Appliances (used, rare and vintage) Archives - Airstream Trailer Classifieds - Airstream Trailers For Sale
I purchased a compartment door for my 66 Trade Wind from Airstream Trailer Supply in Salt Lake City. They have some used parts for sale.
Link: 1969 and up side compartment door
Our old friend e-bay is also a good place to search.
Lastly, search for RV salvage yards that might have Airstream parts. I understand there is one in Waterloo, Iowa, that others have mentioned before.
Hope this helps. And again, welcome to the vintage Airstream hobby. Post some pictures of your old trailer and have fun getting it ready for traveling.