I got one from Dynasty Mattress off Amazon, and it's great so far. I got it because they specifically advertised it as being more of a firm memory foam mattress (same density as a lot of Tempurpedics based on what I read). I went with that for two reasons. The first is that I needed better support. The second is that I've heard a lot of reports of memory foam mattresses losing shape over time, and apparently the higher density foam helps reduce that (good reports from this brand, anyway).
My friend got one from Wal-Mart, and it's served him well. I remember it being on the firmer side when it was new. No idea how much it has softened up or how well it is still holding up after about a year (seems fine from what I can tell).