Available all over, but... please purchase NSF listed, Coconut shell based acid washed GAC. It is typically sold by the cubic foot, or 2 cu. ft bags. There are a lot of garbage GAC's available and they may save you a few cents...
We have tested some of the low grade GAC and some of the stuff found in it is not appropriate for dicsussion on a family friendly website.
Spend a few extra dollars, get a name brand, or at least a NSF or WQA listed carbon.
Centaur, Joacobi, Spartan, Cat-HAC, are just a couple names to look for. Clack dorp distributes a very high quality product.
Hope this helps.
Note, I beleive Centaur is Bituminous coal based, but considering the source, we trust it and it has proven to be an excellent product for many years.