Hi Gdhl8,
I had the chance to spend a few hours looking in the Mankato, MN area and talking to some of the retailers there and I have to say there are not a lot of choices there I would consider but there are a few. The better possibilities I’m aware of (out of almost 20 retailers that I looked at and/or talked to in the surrounding area) and the brands they carry I would consider (subject to knowing all the layers inside the mattress) are …
http://www.bedpostfurniture.com/mattresses/ Mankato, MN. Carries Symbol, Therapedic. Knowledgeable and helpful and has some of the largest latex selection in the area (Therapedic). Well worth talking to and has decades of experience in the industry and good knowledge about mattress materials.
http://mankatomattressman.com/ Mankato, MN. I talked with Justin here who is the owner and he carries Corsicana (low end promotional only), Englander (including latex) and Restonic (including their Tempagel mattresses) and understands the importance of educating his customers about the materials in his mattresses. He is focused on providing good quality and value and building his reputation in the area and would be well worth a visit.
http://www.thefurnituremart.com/locations Medford, Shakopee, and others in MN. Restonic (typically carries gel and latex).
http://www.3daymattress.com/ Owatonna, Austin, MN. Restonic, Symbol, Spring Air, Anatomic Global, Ecosleep. No latex (because of the area) but does have gel memory foam and is knowledgeable and open about his mattresses.
So outside of Minneapolis these are the options I would consider within about 50 miles of you. The Minneapolis / Rochester, MN list has more options and is in post #2 here for those in the area that are willing to travel farther.
Phoenix