I expect any pet shop would sell activated carbon for filtering the
water in aqariums.
Activated carbon is really fairly simple to understand. It is made out
of baking natural plant materials or coal in the absence of air to
produce charcoal. This charcoal will have gazillions of tiny pores in
it (from the plant cells that were baked), or gazillions of tiny
microscopic cracks in the coal. These pores or cracks greatly increase
the surface area of the charcoal, and organic matter flowing through the
charcoal will be "adsorbed" onto the surface of the charcoal.
The German company "Siemens" is a major producer of activated carbon for
use in the industrial water filtration systems they make:
'Activated Carbon' (http://tinyurl.com/bpbztnz )
and I expect that if you e-mailed them asking for advice on what kind of
activated carbon you need for your filter, they'd help you out. You
could ask them who sells their activated charcoal in your area, and
perhaps buy some from them for your filter.
Alternatively, simply Google "activated carbon", and you'll find plenty
of companies selling the stuff.
I've been told that charcoal made from roasting coconut shells makes for
excellent activated carbon because coconut shell is a very hard "wood"
and that means the cells are much smaller than normal hard or soft wood
cells and that results in a finer "filter", but I don't know if that's
true or not. Other woods, like ebony and cocobolo are harder than
coconut shell, but coconut shell doesn't have any other useful purpose
for which it can be sold, and so I expect they use it for making
activated charcoal cuz it's cheap as dirt.
Also, my understanding is that activated charcoal is also called
"catalytic carbon" because it's a natural catalyst, and that leaves open
the possibility of re-activating the charcoal by baking it in an oven so
that the organic matter adsorbed to the catalytic carbon surface reacts
with oxygen and gets burnt out of the charcoal, thereby cleaning that
carbon surface and effectively "re-activating" it. I've heard that
activated charcoal can be re-activated, but I don't know the process or
how effective it would be. Maybe talk to someone at Siemens about
re-activating your existing charcoal. Perhaps even the packages of
activated charcoal you buy at a pet shop would advise you on how to do
this, or you can just google "re-activating charcoal" to find a
procedure to follow.
If you try to re-activate your charcoal, some of the carbon is going to
react to form carbon dioxide and be driven off as a gas, so you're going
to need a bag of pet store activated charcoal anyhow to top up the level
of charcoal in your filter.
Hope this helps.