Fortunately for you, your part of the country is Airstream Heaven. Texas, New Mexico and Arizona tend to be homes for lots of Airstreams used primarily by Snowbirds - retirees who live in the north during summer and travel to the hot dry southwest during the colder months. When age, illness or death intervenes, the airstreams come up for sale. Many have sat in the sun which is murder on the clearcoat, but without the road salt, rain, cold weather and hail storms of the north, the units can be in very good shape. Bigger units are usually less expensive per foot than smaller ones. Of course, you need a more robust tow vehicle and may find that your unit is too big for a state or national park.
Enjoy the looking part as much as the finding. Do you know what a "squarestream" is? And don't overlook the Argosy - or white palace - the other Airstreams. Use the "search" tool on this website and look at the models of www.vintageairstreams.com
Good luck and good hunting.