I don't know how far you are from the big city? Cause you could just open the yellow pages and look under foam or spounge.
Get yourself some orthopedic foam in the 3-4" range; your hinie will love yah. Oh yah, it's cheap too!
Headliners: What type of material were you thinking of using?
Nylon fiber carpet with Nylon backing shouldn't be a problem. Something in the 40 oz. range would be nice. And this you can get from your local carpet retailer.
Once bitten, twice shy