Local stores near Knoxville aren't the only way to shop! You can also find name brands from online retailers and get them shipped right to your door!
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 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!
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.