It's a custom window that is specific to Airstream. Just like windshields for trucks, local dealers don't seem to stock them. It's a high cost "part" and they don't move very fast. That pretty much keeps them out of the stock room. If you have any doubts about this, chat with the stock room manager next time you are at a truck or RV dealer ....
The process is going to involve ordering the part from Airstream. They aren't likely to sell it directly to you so indeed a dealer will be in the mix. For whatever reason, there are a lot of dealers who only order parts once a week or once a month. They also seem to expect that a "normal delivery" will be two or three weeks. Indeed that all strikes me as more than a bit odd (to say the least).
Best bet - give JC a call and see what they say. If nothing else, they might be able to light a fire under the local dealer.