Can be obtained from drilling supply companies in a pinch. Or you can obtain more specialized bentonites such as calcium montmorillonite. Some bentonites are good for taking internally for detox. There are apparently magnetic clays as well which have more drawing power. To my knowledge, bentonite goes negative when wet and has a pull on ions.
Regarding clays perhaps see:
http://www.tuberose.com/Detoxification.html (scroll down to the section about clays);
http://www.eytonsearth.org/
Also, you might contact
American Colloid via email or telephone to ask them the price on clays if you want bulk (50lbs...200 lbs..a ton...).
Clay baths or epsom salt + baking soda baths can also help with radiation detox.
Dont overlook Pascalite for internal use (not cheap and not necessarily readily available in bulk).
The point here, if a nuke goes off, radioactive materials can apparently be removed from the body with clays just like other toxins and heedless suffering need not be experienced--not to mention the buffering properties of the bentonite. As for myself, in case of a 'nuclear event', I'd prefer body coverage with wet bentonite clay to just a bath.
You might find this interesting?
http://www.unitedstatesaction.com/nuclear_safety.htm (he/she is pretty blunt and straightforward).