We got ours from Craigslist. $180 for the two, which were used in a guest room for a month (so basically new). When I originally looked at the Mattress Firm listing (that's where they originally came from) it said memory foam but now when I looked it said "2 inches of memory foam and six inches of orthopedic foam." They're really firm, but I can sleep on anything, anywhere, in any position, so they work just fine for me.
The best bed I ever had was passed around in my family for 30 years. It was rubber foam. Felt like floating on a bouncy cloud. But moving it was such a pain! It probably weighed 150 lbs. and went completely flaccid when picked up on its side. I had to give it away when my family refused to help move it from my last house. If you could find one of those it would be excellent for the skoolie chop-up. Although when I tried to find a new one it seems they're pretty expensive. Couldn't find any for less than $1000 ish.
My build thread:http://www.skoolie.net/forums/f11/build-thread-for-haulin-oats-and-goats-11237.html#post113500
A gal, a guy, three cats, two dogs, one rabbit, and one goat, traveling the country together.