Hi David.gruenberg,
If you are looking to buy a new mattress then the best place to start is post #1 here which has all the basic information, steps, and guidelines that can help you make the best choices.
If you are looking to just buy foam then you could use a local foam shop or some of the online sources here. Make sure you deal with a reputable dealer that is knowledgeable and transparent about what they sell.
Polyfoam comes in a very wide range of quality levels which are based on the density of the polyfoam. Higher density polyfoam is more durable than lower densities. The lowest quality foam may only last you for a month or two of regular use while high quality polyfoam can last for many years.
Phoenix
ADDED PS: I see that dn posted essentially the same thing while I was writing my post … thanks dn 