I got mine at the pet store, in the fish tank section. It's packaged as Activated Carbon, but on the side says 100% charcoal. It's quite granular, I had to run it through the coffee grinder a couple of times to get it as powdery as I'd like. I mix it in some soap batter at thin trace, then strain that through a mesh seive as I pour it back into the rest of the soap.
By the way, if you do that, kiss your coffee grinder goodbye, it'll only be good for soap stuff, the charcoal gets into every nook and cranny. Although I've heard it's good for flatulence.