Check out buyfoam.com. I happened to come across them, while searching for a decent topper. You can customize to any thickness and density. We have never used them before and decided to give a try, so I can't comment on their business practices. I know one thing for sure, it's much less expensive then buying from a big box store. Good Luck!
2012 Nissan NV2500 5.6-liter DOHC 32-valve V8