Hi shotgun233,
Welcome to the Mattress Forum! 
I did a quick glance through the list of members here (I think highly of them all) who offer items online, and I found:
Mattresses.net offers latex and spring base cores.
CozyPure offers a 6" latex core product.
Latex Mattress Factory offers a finished 6" latex core mattress.
MFC offers polyfoam 6" cores. You’d have to check on shipping to the USA.
[url={admin note: discontinued product}]My Green Mattress[/url] has a 6" core latex mattress.
Sedona Sleep has a polyfoam core mattress
SleepEZ offers a 6" finished latex mattress.
You may also wish to take a look at the vendors in the component post here.
Phoenix