You can buy boxes and bags of carbon from Ebay or any pet shop. You might even find a chemical supply company that sells it.
If you get carbon, try to get Activated carbon or Highly Activated carbon, because they are better quality than plain carbon.
If you buy carbon, put it in a fine mesh net and rinse under tap water to remove any black dust, then put it in the filter.
Fine granules of carbon are better than big pieces of carbon.