Hi luvsleep,
If you have the Tempurpedic Original then it would have 2.8" of their firmer 5.3 lb memory foam over a 5" polyfoam base.
If you have the Tempurpedic Classic then it would have the same 2.8" of their 5.3 lb memory foam and another .8" of a softer version of the same density memory foam over this and the same 5" polyfoam base layer.
The Tempurpedic memory foam is firmer, more temperature sensitive, and slower responding than most of the current memory foam on the market but the closest you would likely come in “general” terms would be about 3-4" of 5 lb memory foam over a 5" or 6" firm polyfoam base layer but this would depend to some degree on the specifics of the memory foam that was in the mattress because there are many different types of memory foam even of the same density.
There are of course thousands of memory foam options available both in-store and online, but that’s a much too large list for any one site to maintain. A good place to start your research would be the members here of this site. In addition to this list, post #12 here has some other online memory foam options for those who are looking for a memory foam mattress.
Phoenix