I've looked for a 1968
replacement door since 2010 and have not found a good one.
To achieve your goal, you need a door and
the door jamb. Without the corresponding cast jamb, a 1968
door only, does your concept no good. The jamb is built into the trailer frame/shell and will have to be removed and shipped as well.
You already have a door and jamb on the curb-side. There are hundreds of shade-tree, back-yard, driveway Airstream fabricators in the USA that would attempt the swap, and do a great job, even better than the Airstream factory. Cheaper and faster to import one of them to Australia for a few weeks, than finding and shipping a 1968 a door and jamb.
I've seen those Mad Max Movies and that tells me that there are already excellent metal fabricators in your Out-Back. There may also be an air-frame mechanic near you looking for side-work.
Isn't there a historical/vintage vehicle variance to the code?? or do you want to walk through from side to side??