Hi musicus4,
Just in case you haven’t read it yet … the first place to start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best choices. It also includes a link to a list of the better online memory foam manufacturers and retailers I’m aware of.
Firmer memory foam mattresses will often use 5 lb memory foam in relatively thin layers (2" - 4") over a firmer polyfoam support core but there are so many varieties of memory foam that your own personal testing is the best way to know whether a mattress is a good match for your needs and preferences. If you are looking at online choices then more detailed conversations with the retailer or manufacturer is the best way to make the most suitable choice since they will know more about their mattresses than anyone.
Some of the better options and possibilities I’m aware of in the Minneapolis area are listed in post #2 here.
Phoenix