You can try garden centers in your area, ones that sell pond supplies. Ring up first though as some charge stupid prices. You can use pretty much use whatever you want, people on this site like the plastic pot scrubbers, lego bricks, loose change...
Ebay and amazon aren't a bad place to try just look at filter sponge you should get 50cm x 50cm for about a fiver each, probably less. I'm currently using coarse 10ppi as prefilter on the intake, one 20 ppi in the first tray and that's it. You don't want to pack your filter full, flow is king.