I can't help you find a door except to say the used doors do come up for sale every once in a while, but you have to be diligent searching all possible relevant sources (and then move fast), including many vintage Airstream related Facebook groups. Attached is a screen shot of the last such Facebook ad I saw, just 3 weeks ago (unfortunately already sold).
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And back to the original question, I believe the main reason Airstream began using suicide doors in the early to mid 1950s was to avoid the door hitting the frame of an open window. There were a few exceptions to this "rule" at least as late as 1956
(especially the 1954-56 22' Safari, but there were other rarer cases, too) where a non-suicide door was used that could hit an open window immediately in front of the door, but Airstream always (or almost always) used a front hinged, non-suicide door when there was no opening window forward of the door (basically all trailers shorter than 20-feet and some 22 footers at at least into the 1960s).
In the decades since, the presence of factory installed awnings has also become a factor so that the door would not hit an awning arm. In some cases, like the modern 28-footers with an awning arm immediately in front of the door and an opening window immediately rearward of the door, they had to make a decision to violate one of these rules. The current rear door 23-footers are opposite and have a non-suicide door than can crash into an open window immediately forward of the door. It seems that now that having the door avoid the awning arm takes priority over avoiding hitting an open window when both conditions exist on the same trailer.
Most current Airstreams with a window immediately rearward of the door have small stacked non-opening windows in that location to eliminate the possible crash condition, but not the 28. The opposite is true for most rear door models that have an awning arm immediately aft of the door, so non-opening stacked windows immediately forward of the door, but not the 23.