I was trying to find blue foam at Petland today, and since they didn't have it, I went with fluval. Working really well so far, though it's a bit pourous so I have to cut it juuuuuust right. However, once I get the correct size, it's working like a charm.
Cost $8 for two rectangle not too wide strips of it, but I believe that since I reuse everything till it dies, that it will last a long time.