If you've never shopped overstock.com before, I highly recommend it. I have bought a lot of things from them- everything from bedding to books.
I purchased THIS Memory Foam topper a couple of years ago and we LOOOOVE it. It is still a great price, you can't beat 85% off!!
This site has great customer service and super fast shipping. Check it out. :)
Oh, and, If you use Ebates, you can get 4% cashback. Link through EBates HERE.