I recommend
http://www.us-mattress.com/ if you know what you want. Dependable, free delivery, half the cost of regular mattress stores, and free exchanges if it doesn't work out for you.
Anecdote:
When I went mattress shopping a few years back I visited all of the stores in town, like SleepCountry and the like. They were very friendly and helped me find what kind of mattress I liked.
Then I made the mistake of checking the internet for prices on those same mattresses that I liked in the store, and they were typically 40-50% cheaper online. I went back to the stores and explained that I was happy to pay a little extra beause they have overhead and a showroom and helpful sales people, but their prices seemed awfully high. They wouldn't budge, and were very critical of my online price comparison shopping, as if I had somehow betrayed their trust.
Which is bullshit. They're a business and they want to make money, and I would help them make money if they were charging 10 or 15% more than online retailers. But I'll be damned if I'm going to pay TWICE AS MUCH for the exact same mattress. So I ordered from us-mattress.com/ and haven't regretted it for a moment.
Also, for people who really like memory foam mattresses, Costco sometimes has quality memory foam mattress pads for less than $200. Unless your current mattress is super saggy in the middle, I think this solution is just as good as the $4k memory foam mattresses.