Obey these rules and you will have a used fwc someday
1) Reasonably priced used FWC's go quick, usually within hours of their posting on craigslist. Best place to look is on craigslist in your area, or use crazedlist (google is your friend, as is firefox) to search a wider area. Have a pile of cash sitting around and be ready to pounce. By far the best resource is the used camper postings on WtW, go to the last page of the post everyday and check it, several times a day would be better.
2) Make sure your older 4X4 truck can take the camper you buy. This is very important. There is extensive posts on this subject on WtW, and the fwc website has info as well. You can modify (ie, build a platform) for some older 4X4's to take certain campers, again, make very sure you don't buy something that won't fit on your truck.
3) Most "cheap" FWC's are located in Colorado as that is where the company was based back in the day. These are mid eighties or earlier, often need significant work/rehab to make livable. It is not uncommon to see sub one thousand dollar older Grandy's on craigslist. Every once and awhile someone who has no idea what they have will post a newer fwc for a ridiculously low price, this is very rare, roughly akin to sitings of the Virgin Mary.
4) ATC hasn't been around so there aren't many used ones for sale, and they tend to retain their value, as do FWC.
5) Other pop up campers are not built for serious off roading (ie, Palimino, Outfitters etc). Not saying they are bad but don't expect them to hold up or last like an FWC or ATC if you plan on going to remote locations on washboard roads. They can often be found for a lower price and have more features than a FWC, but will weigh more too.