Word of mouth.
I was on the brink of buying a brand new Scamp when my husband came home from work and said he'd mentioned the purchase at the lunch table and someone said "Ya know, I think so-and-so has something like that he's looking to sell".
Called the fellow the next day, he said he had a
Trillium 4500 for which he was asking $1400.00. I decided that if my Kia could tow it, I'd buy it. It could, and I did. This was back in 2005, but even then prices were overheated at specialty sites like this one. There are still "willing buyer/willing seller" deals out there...last year I tipped a seeker here off by P.M. about a Scamp I saw
for sale over in Eastern Washington at a VERY reasonable price. Just as with my own deal, both parties walked away completely satisfied with the bargain.
Just tell everyone you know what you're looking for and you might be surprised what turns up.