They're too soft for seats, if that's what you're thinking. I researched that heavily last spring. You need high-density foam for a seat or you'll sink right through it.
To answer the question, they can be had at any bed retailer for $40-60.
if you had a sheet of it, 2" or so, you could trim down the main foam and use this on top. the factory foam is very rigid actually and would act as a good foundation.
One of my other-mothers got me one when I was lining the doc's pockets. It's nice if you sleep on your back. It sucks if you roll around at night though. It's also WAAAAY too soft to use for seat foundation.
