I know that if you look outside of fish related places, you can get large amounts of carbon for way cheaper than an LFS would sell it for.
The problem, which may not really be an issue, is that most of that carbon is sold for air quality purposes, and although it should still absorb stuff in water, it may not do it as effeciently as the carbon designed for water.
if you wondering the difference, its mostly in how it is activated (steam, fire, high pressure hot air, etc) which determines weather it will better absorb stuff in air, or water.
sorry i sort of went slightly off topic, or got too into it, but your talking bulk, and im thinking 50lb bags which i know are cheaper when purchased places outside of an LFS.