Hi itbgc,
The closest forum lists to you are the Kalamazoo / Portage list in post #2 here and the South Bend / Fort Wayne list in post #4 here
I also did a quick scan around St Joseph and there are a few retailers that may also carry latex mattresses or latex hybrids closer to you. I’ve listed the manufacturers they carry (according to their website at least) that make latex mattresses or hybrids but you would need to call them to make sure they have some latex mattresses on their floor and also to make sure that they can tell you the specifics of what is in their mattresses (layer by layer) so you will know what you are testing.
http://erhardfurniture.com/ Berrien Springs, IN. Symbol, Therapedic, Restonic.
http://www.yourmattresswarehouse.com/ Niles, MI. Sleep Logic, Symbol, Therapedic, Dr Breus (they have some latex but don’t normally provide the specifics)
http://www.imperialfurniture.net/ Dowagiac, MI. Lady Americana
http://countryhousefurniture.net/ South Haven, MI Therapedic
http://www.abcwarehouse.com/default.asp South Bend, IN. Restonic, Spring Air
http://bobmillers.com/default.asp South bend, IN. Symbol.
Phoenix