After doing about a month of research (The Mattress Underground is a good forum) I bought a latex mattress from SleepEZ. Had it for a couple of years and been very happy with it.

One consideration to keep in mind is that if you sleep in the middle of your bed you won't want to get the split layers. They make assembly easier and allow for more granular customization, but a friend of mine who got the same mattress feels the crack between them where the layers separate slightly.