If the part is availabe from Airstream then it can be ordered from any Airstream dealer. There is a list of dealers on the Airstream web site. If it is not a current part or you want to try to save a few $$ then your next option is a boneyard. On the vendors forum there is a place called Colaws that has some Airstreams of your vintage. Another option if you have no luck on either of these is to call Andy at Inland as he has a source for the door locks new or old.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato