I've always found Bentonite clay online. The best prices I've found anywhere are here. http://search.store.yahoo.com/cgi-bin/nsearch?catalog=vitaglo&query=bentonite...
There are also many other sites that sell Bentonite clay. Just go to Google and do a search for Bentonite clay.
This powder will have to be hydrated, but I just take mine and add distilled water to it in a covered plastic container, use a hand mixer to get it to a yogurt consistancy and use it as needed.