Typically, it is a medically oriented product and can be found in just about any store that has home health remedies such as cough syrups, tylenol, aspirin etc. Try your local drug chain, it is also in first aid kits and lists with ipecac syrups indicated for poison ingestions not linked to petroleum products.

"It should not be that expensive, most likely about 4 bucks for 8 oz bottle....just a guess but then again I used to buy it by the case not the bottle in my line of work. This type is fine particles that you can pour like salt. To me this is easier to work with when you make your filter."
Where to buy Activated Carbon? - The Garden's Cure

You may need to transfer after a while, a computer case might get kind of snug on a mature plant, good lucK!