I think its readily available at your local health food store as well as other paces on the net. I had been searching for it after my question on black colouring for soap. I saw some links to ebay too.
I bought mine at my local health food store. Since I was only making a trial batch, I didn't want to buy a large quantity of it, so I bought a small bottle of Nature's Way Herbs Activated Charcoal in capsule form. The charcoal in the capsules was very fine and did not need any grinding at all. It was like talcum powder in consistancy. I found through measuring that 5 of the capsules equalled 1 tsp, which was perfect for my sample batch.
I get mine in the dietary supplement isle of most stores like Walgreens, WalMart and Target. It's It comes in pill and capsule form. I use the capsule form and break them apart so I don't have to crush it. It's used as an anti-gas pill so it could be in that area of the pharmacy as well.