Go shopping.
Do NOT buy a mattress at the first place you go to. Hit at least three places, tell them the same requirements (side sleeper, back sleeper, preference as to firmness, how many pillows you tend to use). See if they all recommend the same brand and type, and then get the best price on that bad boy (ask the last place you go whether they'll price match if theirs isn't the cheapest).
See if they offer to set up your new one and take away your old one, too - that's worth a few extra bucks, to me. If you have a truck, maybe it isn't.