I'm down in Georgia, and 720's are almost non-existent down here. I think I've seen one in person, other than mine.
If you're up in New England, you'll have quite the time finding one, let alone one without rust. They're getting rare on the east coast. I watch eBay and Craiglists across the country, and it really depends on what you're looking for. 2WD trucks come up for sale more often, but I have seen some nice 4x4's, usually between $2500-4500. If you're willing to pick one up from Oregon, Nevada, New Mexico, someplace like that, I don't think you'd have too hard a time finding one with little to no rust and lower miles.
Check out 720 World and Ratsun forums. Some other great places for inquiries.