Hi bbbassoon,
The first place I would start with any search for a new mattress is post #1 here and the information it links to (in case you haven’t read it already).
Some of the better online options for memory foam mattresses I’m aware of are listed in post #12 here. Many of these may be able to give you good suggestions about which of their mattress most closely approximates the Tempurpedic supreme in either quality of materials, feel, design and layering, or any combination of these.
Latex and memory foam are very different materials with a very different feel and response and one is not really equivalent to the other in any firmness level. Because the choice of material is a preference … it’s usually a very good idea to test different versions of latex mattresses or comfort layers locally to make sure you are familiar with how it feels and performs and to act as a guideline for your latex comfort choice.
Other than this the only way to really choose would be in a conversation with them based on any information you can supply about your body type, sleeping positions, and preferences and the “averages” of other people with similar body types, sleeping styles, and preferences which may or may not apply to you. In this case I would encourage you to consider a mattress with comfort exchange options so that you can change your choice if it’s necessary based on your actual experience.
Phoenix