Step 1: A bit about activated carbon and where to buy small quantities.
The History of Activated Carbon
Activated Carbon was first known to treat water over 2000 years ago. However, it was first produced commercially at the beginning of the 20th century and was only available in powder form. Initially activated carbon was mainly used to decolorize sugar and then from 1930 for water treatment to remove taste and odor. Granular activated carbon was first developed as a consequence of WWI for gas masks and has been used subsequently for water treatment, solvent recovery and air purification. The unique structure of activated carbon produces a very large surface area: 1 lb of granular activated carbon typically provides a surface area of 125 acres (1 Kg =1,000,000 sq. m.). Activated carbon can be produced from a variety of carbonaceous raw material, the primary ones being coal, coconut shells, wood and lignite. The intrinsic properties of the activated carbon are dependent on the raw material source. The activated carbon surface is non-polar which results in an affinity for non-polar adsorbates such as organics. Adsorption is a surface phenomenom in which an adsorbate is held onto the surface of the activated carbon by Van der Waal's forces and saturation is represented by an equilibrium point. These forces are physical in nature, which means that the process is reversible (using heat, pressure, etc.) Activated carbon is also capable of chemisorption, whereby a chemical reaction occurs at the carbon interface, changing the state of the adsorbate (dechlorination is an example of a chemisorption process). (You can read more here: http://www.carbochem.com/activatedcarbon101.html )
Activated charcoal is good at trapping other carbon-based impurities ("organic" chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all -- sodium, nitrates, etc. -- so they pass right through. This means that an activated charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter. (You can read more here: http://science.howstuffworks.com/question209.htm )
Over 100 years ago Ellen White, a health reformer & pioneer of the Seventh Day Adventist movement strongly advocated the medical uses for charcoal powder. The modern medical establishment has only recently begun to use activated charcoal powder as the preferred method of treating oral poisonings and drug overdoses: "It is thought to bind to poison and prevent its absorption by the gastrointestinal tract. In cases of suspected poisoning, medical personnel either administer activated charcoal on the scene or at a hospital's emergency department. Dosing is usually empirical at 1 gram/kg of body weight, usually given only once. Depending on the drug taken, it may be given more than once. In rare situations activated charcoal is used in Intensive Care to filter out harmful drugs from the blood stream of poisoned patients. Activated carbon has become the treatment of choice for many poisonings, and other decontamination methods such as ipecac-induced emesis or stomach pumps are now used rarely." (From the Wikipedia entry for Activated Carbon)
You can find it at any fish & aquarium supply. If your concerned that the quality of the carbon from an aquarium shop might not be up-to-snuff, then go to a homebrew shop, or some other source that you are comfortable with. The granule size you'll want should be relatively close to 8 x 16 mesh size or smaller, but NOT so small that it falls out of the holes in your filter housing. I purchased the NSF approved carbon I used for my filters through an ebay merchant (here: http://stores.ebay.com/Carbon-Eze ). No, that's not me, & I don't know them. :) They describe their products well and have a good reputation & that's why I've included them here.