I can never resist a dig at big pharma (aka the grim reaper) so got to say this.
in the UK it's quite difficult to find mainstream pharmacies that carry it cos there is absolutely no mark up/ profit on activated charcoal and it's superbly effective for many stomach complaints. Obviously they would rather you buy some death in a bottle/blister pack....... Okay breathe banana - right rant over
The cheapest and best place to get hold of activated charcoal is any decent fish/aquarium shop.