From the east coast, Welcom To The Forums!
In addition to where you are already searching, take a look at the Airstream dealers and other RV dealer web sites. You never know when or where an Airstream is going to turn up. Plus most of the dealers now have an on-line inventory listing. There are also places in the web like RV trader, etc...
You may also want to take a look here...
Airstream < Airstream Campers | Vintage-Airstream.com
RV TraderÂ® - New and Used RVs for sale on RV Trader
Good luck in your quest and don't be afraid to ask questions.
Also be aware that there may be Airatream owners near you or a trailer that you are interested in that would be willing to make an inspection of the trailer with/for you. Use the resources here in the forums and you won't be disappointed.