There are a few forum lists that cover some of the “other” Columbia’s around the country but none that cover the Columbia, MO area. You are in between the St Louis, MO list in post #6 here and the Kansas City list in post #2 here but both are over 100 miles away.
The two manufacturers I know of in the area that would be worth visiting are …
Furniture Row® Store Locations - Store Hours & Addresses They are a regional manufacturer that makes a range of innerspring mattresses and a “mostly latex” mattress/topper system. They use good quality materials in their mattresses, are fully transparent, and are better than average value. I would avoid the mainstream mattresses they also carry and only focus on their own mattresses.
http://www.jcmattressonline.com/ Jefferson City, Lake Ozark, MO. They make their own brand JC Mattress and also carry Serta and Spring Air (both of which I would tend to avoid). I haven’t talked with them and there are no details about the types of mattresses they make on their site but if you can confirm that they will disclose the type and quality of the materials in the mattresses they make then they would certainly be worth a visit to see what they have available.
Over the course of the next few days I’ll take a look in the Columbia, MO area and do some preliminary research so I can list some of the other options and possibilities in the area as well and list them in this thread.
I took a look around the Columbia, MO area (within about 50 miles) and out of about 30 retail stores and chains these are the stores (in addition to Denver Mattress) and the brands they carry that have better odds of being able to find out the details of what is inside their mattress (assuming the retailer is willing and able to find out for you).
American Freight | Discount Furniture, Mattress & Appliance Store Columbia, Jefferson City, MO and others in the area. Therapedic.
Columbia MO Furniture Store Wholesale Hot Tubs, Pool Tables, Bedroom Sets, Mattresses , Cabinets... Fayette, MO. Fraenkel (Englander and Symbol licensee).
Browse Our Exclusive In-store Furniture Specials - Do Furniture Moberly, MO. Fraenkel (Englander / Symbol), Restonic, Justice
There aren’t a lot of great options within about 50 miles or so and a little longer drive or an online purchase may also be worth considering depending on the results of your research at Denver Mattress or the other local stores. I would also make sure you call first before taking a longer drive to confirm the types of mattresses they have on their floor that you can test (latex, latex hybrids, memory foam, traditional innersprings one or two sided etc)