Local stores near Irmo aren't the only way to shop! You can also find name brands from online retailers and get them shipped right to your door!
The position in which you sleep can play a role in your choice of mattress:
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 high quality mattress isn't cheap, with prices ranging from a few hundred all the way to a few thousand dollars. How do you get the most bang for your buck? In most cases the answer is to choose an online mattress retailer. Physical retail locations have huge overhead, and the online sellers don't play on the same field. Since they all have great return policies, you should definitely try one out.