Airstreams are generally measured including the tongue - which is about 3 feet long, so you'd be looking for a 21 to 23 foot Airstream.
Condition is everything! Especially in a used Airstream and many of the potential problems aren't easily spotted by a first time looker. Also on the west coast, Airstreams are a bit more pricy new, simply because of the cost of shipping them from OHIO. That may also reflect on the price of used ones.
If you have the luxury of time, you should educate yourself thoroughly before pulling the trigger. Join your local WBCCI, visit some dealers, look for AirForum rallies in your area, and generally poke your nose into a LOT of different Airstreams... And while you're at it, look at the "silver kin", Avions, Streamliners, etc. Looking at floorplans won't tell you that the bathroom door is too narrow for anyone with a waist bigger than 24" can't fit through, or that knee room will be a real problem. And what if you're too tall for a twin bed model? First find something you'll really want to keep for a long time, then go looking for that model! Many people move up from a smaller to a mid sized Airstream simply because they want something they can get the most use out of. Rallies, especially restoration ones will be full of people who can show you what to look out for... like rear end separation or bent/broken frames.
Once you've got your homework done, you'll get a better bargain on a gently used Airstream. If you decide to buy new, do it NOW before the camping season really starts. (NOW means this month!)
Good luck, Paula
Today is a gift, that's why they call it the present.