I reuse the original filter media that comes with the filters, if it needs replacing i buy the sponge intended for the filter from my LFS, Last time I got a sponge for one of my large AquaOne filters it only cost about $10.
p.s
I don't bother with filter wool, waste of money and in general, very wastefull to replace every month. I always end up replacing it with a sponge which lasts for years.