Shopping for a good mattress is very difficult. It's not like all the stores carry the exact same models for a particular mattress brand. Thus, it's hard to directly compare one mattress to another between stores.
Initially we shopped around many stores in the GTA looking for our queen sized mattress, and found varying prices. Probably the most expensive the were the big box stores like Sleep Country Canada.
We stumbled upon a smaller mattress store in Scarborough called Mattress Mall. Our last purchase was 7 years ago and we were very happy with our Sealy mattress purchase. Since then we have returned again to buy a king sized Simmon's beautyrest, and also a twin mattress for my son. Our sales person Matt is very knowledgeable and doesn't pressure you at all when you're in the show room. Plus they've moved across the street from their old location to a bigger and better showroom with more models for you to test out. They carry all the major brand names (except Tempur Pedic).
Do shop around a few places before you make your purchase, but we are very happy with our purchase and service with Mattress Mall in Scarborough.