Question about size is really about tow vehicle.. Having something large enough to tow more than 7500# (weight of 30' and larger trailers..) requires a meaningful tow vehicle...
Assuming that is OK, there are 3 primary sources of electronic listings other than E-Bay...
1. The great classifieds right here.... (Rivet tab #4 above..)
2. Vintage Airstream Site:
www.airstream.net
3. RVTrader Online :
www.rvtraderonline.com
These three usually offer more than 100 trailers for sale at any given time. Be prepared to use volunteers or friends or military colleagues to look at trailers out of your area, because Airstreams are becoming (or already are..) a near-national market... Your dream trailer might require a long drive to bring home, but odds improve greatly if you're willing to consider a larger search radius... The E-Bay auction model can work, if you can see and verify the trailer also, but sellers have a tendency to omit certain negative details and there are a number of fraudulent sellers and completely phony deals that pop up from time to time..
Beyond length, the other key variables to consider are: Age, Condition, Repair Costs and Interior Layout... Sleeping and bathing and sitting and eating arrangements vary widely, and you should pick a layout that works, rather than assume you can easily make major changes, unless you really enjoy metalworking, carpentry, plumbing, and cursing...
There are a number of choices in the price range, but be really aware that buying one in bad condition can force $ thousands in expensive repairs, from appliances (A/C, Fridge, Water Heater, Tanks and Plumbing, Stove) to leaks and floor or water damage to upholstery and windows/curtains...
Good Luck!
John McG