"Hehr Standard" windows, which is the brand and model your AS has as original equipment, do come up fairly regularly on eBay. Set up an eBay search for Hehr Standard window and check it daily. It may be there now, or it may take a ~good while~ to see the exact size you need but it will be there eventually. Prices trend from just under a $100 each for a small size Hehr Standard to ~$300 for the largest ones. Condition varies, of course. Btw, although Hehr is still a major player in the RV windows business, no new Hehr Standard windows have been made by them for a VERY, VERY long time, nor did Hehr keep any of the dies to make replacements. Another source may be by contacting the reputable rebuilders like Frank Yensan at http://frankstrailerworksblog.blogspot.com/
One more thought, occasionally, on the eBay Hehr Standard results you will see a post by a person who rebuilds them for about $250-$300 so if you contact that person, they may be able to supply you with what you need, and if you have any part of the missing window, then that can only help. Good luck!