Jason,
Just buy the AC material and cut it to fit. Works great. I use the cartridge AND the AC sponge material, but I don't replace the cartridge until it's falling apart.
In some tanks I'm using (can't remember the name) but it's the same blue material the outer layer of the cartridges is made of. It's thinner and great if you don't have much space. It's also cheap and you can buy a huge bag of it for like $6 at an LFS.
Roan