Yeah. go online, its called filter floss. Chances are your LFS
has some corner filters running in some of the tanks, and I bet they have the white (at least when its new) floss in it. It is somewhat cottony, kinda like the stuff they put in the top of the tylenol bottles? It isn't cotton, its some other material, but when you see a fluffy bag stuffed with it you will instantly recognize what it is.
I cram it into the aquaclear below the biomedia, and in the fluval I cram it into a media basket below the biomedia. Its just another layer of mechanical filtration, although like anything in the tank, it gets colonized. Way back when, before the ceramic porous biomedia, filter floss and carbon was your biomedia too. But with biomedia elswhere, it is disposable fine mechanical filtration.
It is sold in sheets that you can form cut to any shape you want, or in big fluff bags that you just pull off what you want and cram in.