Go to Walmart or Fabricland or Michael's and look for 100% polyester sheeting used to make quilts. It should be 1" thick or less and be flat sheets (not balls). It is cheap, but it is not reusable.
You can also buy filter pads made by several different companies that are cut to size and washable like filter socks. They come in 50-300 micron densities. Here is a link to the Coralife Pureflo media:
http://www.petstore.com/Coralife_Pur...AQFMME-vi.html