<A HREF="http://www.crazypc.com/products/fans/fanaccessories.htm" target="_new">http://www.crazypc.com/products/fans/fanaccessories.htm</A>
I typed in Case Filter in Google and this was the first site. Look at the bottom of the page. I've seen them at CompUSA also. Some are a fine metal mesh, so they can be cleaned and reused.
I too once had money, then I got into computers...
<A HREF="http://service.futuremark.com/compare?2k1=6607283" target="_new"> 3DMark </A>
<A HREF="http://www.anandtech.com/mysystemrig.html?id=24383" target="_new"> My Rig </A>