Hi Jim,
Just in case these are close to you … there are a couple of manufacturers in Central Montana that sell latex mattresses (and other types of mattresses as well) that may be worth considering. They are …
Denver Mattress® - The Easiest Way to Get the Right Mattress Billings, Great Falls, MT. They are a regional factory direct manufacturer that makes a range of mattresses including their iChoice which is a “mostly latex” mattress with either a memory foam or latex topper. They disclose all the information about the materials they use in their mattresses and are typically good quality and value (and certainly better than mainstream brands). I would avoid the major brands they also carry.
https://www.mattressmill.com/ Bozeman, MT. They are a local manufacturer that also makes a wide range of mattresses including latex and component mattresses with a zip cover (both innerspring and latex) that can be customized and have some good quality/value and that would be well worth including in your research.
http://www.paylessfurnituremt.com/ Great Falls, MT. Carry Therapedic latex, memory foam, and innerspring mattresses (no longer carry Restonic although they are made in the same factory)
http://bobcatmattress.com/ Bozeman, MT. Carry Englander latex, memory foam, and innerspring matttesses.
http://fullhousefurniture.com/store//index.html Missoula, MT. They are a retailer that also carries a range of mattresses of many types including some smaller manufacturers. They seem to have relatively good value and it may be worth checking to see if they offer any all latex (it looks like they only carry latex hybrids).
http://www.modory.com/ Missoula, MT. Carry OMI mattresses which make range of latex mattresses (including organic) that use very high quality materials but they are also in a higher budget range than many other latex mattresses so make sure you make some very careful value comparisons here.
A little further away there is also a list for the Spokane and Coeur d’Alene areas in post #4 here.
Hopefully … one of these is close to you and has a suitable mattress or at least would be close enough to be able to do some testing.
Phoenix