I have one of these and it’s been fantastic.

Seconding Fred's Beds, I got mine from there and it was good.

Alternatively, a friend got this mattress (http://www.amazon.com/Sleep-Innovations-SureTemp-Mattress-Warranty/dp/B003CT37L0/ref=sr_1_1?ie=UTF8&qid=1367893010&sr=8-1&keywords=foam+mattress) from amazon and she really likes it. I think it's weird to buy a mattress before trying it, but her argument was 5 minutes of sitting on a mattress in the store vs hundreds of amazon reviews may give the amazon reviews an advantage over a quick judgement in a store. So maybe try online too?

http://www.amazon.com/Sleep-Innovations-SureTemp-Mattress-Warranty/dp/B003CT37L0/ref=lp_2457301011_1_1?ie=UTF8&qid=1370582595&sr=1-1