lovemymattress.com can make you any size mattress you want either out of high density foam, innerspring, or memory foam. I have a couple of foam mattresses from them and they've all worked great for me. And they're not too spendy.
Replaced my WalMart tarp with a Kelty car tarp. Quite a bit more pricy but worth it. Fits well even without a roof rack and offers plenty of options for setup.