If your neighbors and local goverment types don't care, and it's semi-permanent, may I suggest the larger models, from 31' to 34' long.. They are roomier, have better bath facilities, and are relatively less expensive than smaller units because fewer people can tow them.. Even if you have to borrow a truck to get it on your farm, it would be lower overall cost, and roomier... It could also handle a few more guests, with a little more privacy... I'd also recommend newer than 1975-ish, to limit risk of major repair and older or obsolete appliances and plumbing problems.. (Though a 1980 model could be scary-troublesome...)
And who knows, you may get the urge to buy a big honkin' truck and drive off on a trip yourself one day...
The best places to look are:
1. Classifieds here..
3. airstream.net classifieds
Be very wary of E-bay, as there are a lot of frauds and scams out there, though some have been succesful...