There aren’t any stores in or close to Steamboat Springs I’m aware of that would be worth visiting. The closest forum lists are for the area around Denver in post #2 here and for the area around Fort Collins in post #4 here.
Foam Source in Boulder also carries a range of component latex mattresses and would probably be worth a phone call and/or possibly a visit.
If the slats are 1x3 slats and they have little to no flex and good center support to the floor then it’s unlikely that a few additional slats would make a noticeable difference in the firmness of your mattress. The extra slats would provide a little extra load bearing surface area but they wouldn’t be “necessary” to provide suitable support for the mattress either.