Unfortunately, you're not alone in your search for perfect sub-20' Airstream... See post below for link to RVTraderonline, and you might also try www.airstream.net
to see what classifieds look like on Vintage Airstream site. Many come and go within a week, so it pays to have funding plans in order, and decision-making hat firmly planted on head... There was an 18 or 20' Argosy on RVTrader site, with very minor body ding, but I can't recall location..
There is also a section of this forum to list trailers for sale, though there are not many items listed, probably due to challenges of getting threads started...
You may wish to rely on other Forum members to do some local inspection/checkouts if you find one away from Portland, though usual warnings should apply. A number of posters have bought as far as 2,000 miles from home and dragged back home to begin loving restoration...