Hi B0T0X,
The first place to start is the tutorial post here which has all the basic information, steps, and guidelines that you will need to make the best possible choices. If you are looking for a memory foam mattress that is sold online It also has a link to a list of some of the better online memory foam retailers and manufacturers that I’m aware of.
You can read more of my thoughts about BedinaBox in post #2 here. I would be very cautious with any memory foam mattress that didn’t meet the minimum guidelines listed in post #10 here.
I’m happy to help you with “how” to choose but the specifics of “what” to choose will depend on either your own personal testing for PPP if you are purchasing locally or on more detailed conversations with online manufacturers or retailers who are more familiar with their mattresses than anyone else and can use their knowledge and experience along with “averages” based on their customers that may be similar to you to help you make the best possible choice out of the options they offer (see mattress firmness/comfort levels in post #2 here).
Phoenix