Local stores near Spearfish aren't the only way to shop! You can also find name brands from online retailers and get them shipped right to your door!
Weight and body shape are a major thing you should consider in your choice. Your experience with a mattress may be completely different than someone who weighs significantly more or less than you. In general, heavier sleepers will need more support from a mattress to be comfortable, than a lighter sleeper.
The firmness of a mattress refers to how soft it feels, and is a somewhat subjective measurement. Don't confuse this with the support it provides, as that's more a measure of how well it holds your back in position. Firm mattresses may provide great or horrible support, and the same is true for softer options. Find a blend that works for you!
A good quality mattress will last many years, but unlike a diamond - they aren't forever. A general rule of thumb is that you should consider replacing your mattress after 8 years or so. If you find yourself waking up with joint pain, have trouble sleeping through the night, or just aren't feeling rested after a night's sleep, you should definitely think about a replacement.