in the US you should be able to buy Oil-Dri directly from Walmart... I found a 20lb bag or so there for around $3.....
http://en.wikipedia.org/wiki/Diatomaceous_earth more than you ever wanted to know about Diatomaceous Earth. And according to some of the info on Harry Harrington's site (link in AlainK's post) the murky residue can be used as an insecticide.