I've used foam from Hobby Lobby before. You can easily cut it with an electric carving knife - you know, the type that is supposed to be used for cutting ham or turkey slices. They work great for cutting precise and complex shapes in polyurethane foam.
More recently, I saved the foam topper out of my bed when I replaced the visco-elastic foam with newer foam. That stuff's harder to cut than normal polyurethane foam because it's a bit tacky and it drags against the knife. It's very similar to the foam that they use for ear plugs.