Local stores near Missoula 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.
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.
The position in which you sleep can play a role in your choice of mattress: