we buy most things online because of the additional discounts we get using coupons and routing through web sites. This includes buying online and picking up on the store. I have a thirty dollar check I need to cash. got it just for clicking through to Walmart etc after first visiting a shopping site.
For a mattress buy a firm with no pillow top etc
Sam's or Costco etc is the place for that usually
A good firm mattress will last much longer than the pillow top. The trick is to then also buy a thick, quality memory foam topper. A good one will last a year or three. Getting a new topper is not expensive and basically gets you a new mattress.
There are sites and forum s dedicated to these mattresses and toppers.