Rinse the carbon granuals in clean water before you use them and bake them in the oven to get them dry again - then they wont be so dusty.- - - - cause the dust from the carbon can create a bit of a mess.
5lbs
http://www.bghydro.com/BGH/itemdesc.asp?ic=AOCMACR05&eq=&Tp=
15lbs
http://www.bghydro.com/BGH/itemdesc.asp?ic=AOCMACR15&eq=&Tp=
25lbs
http://www.bghydro.com/BGH/itemdesc.asp?ic=AOCMACR25&eq=&Tp=
You will soon realize making your own carbon filter isnt really that "cheap" of an alternative. You can buy Dutch Breeze carbon filters with 12 lbs of carbon for like $90. And after a few years, you can replace the carbon in the filter if you really want, or just buy new ones.
Its so expensive because its carbon that is specifically for odor control. You can get all kinds, some is for water, some for air. And even then there are many different grades/quality.that site charges way to much i got 10 lb for $25