I like the vision, but the logistics might not be practical. As has already been noted, most of the owners here would rather be dragged ten miles by their teeth - they are very protective of their beloved Airstream.
Secondly, these trailers get very hot in the summer without hook-up to a 30 amp power supply which supports the air conditioner. If you are thinking of parking them in a farmer's field, you could arrange for a power supply. There are many companies that offer utility-support services for special events, but it would add to the cost factor.
If you cannot get enough Airstreams, have you thought of regular vintage trailers? (Non Airstream). They would look equally kewl, and there may be more owners willing to rent. The downside is the fact most of these oldies do not have any air conditioning or full washrooms. Your guests will need access to a shower - you could rig up an outdoor shower!http://pinterest.com/bellasmile2/outdoor-showers/
That would be an experience for your guests. (BTW, I am involved in event planning and it really is all about creating those unique experiences!)
I know of one vintage rental company in Bend, Oregon (if you mean Portland, Oregon) called "Cowgirl Cabins."
Looks like their website is down (Cowgirl Cabins). They may be out of business. I believe it - there are lots of issues/complications in the vintage trailer rental market.