+1 Biff. I would highly recommend getting ROX 0.8 carbon. It's one of the best carbons you can buy.
I would avoid cheap carbons. They may not have the proper pore sizes and can get clogged fast or may contain high amounts of phosphate.
If you want to avoid buying from a "fish" place, look for a medical supply store that sells carbon for kidney dialysis machines...but that may not be the cheapest route to go.