on a normal purifier the inlet and outlet come out at the top but thats just a bit too difficult for DIY purposes. so the inlet would be put at the bottom and the outlet at the top to produce a slow upflow through the carbon, both need to have a fine mesh filter to stop the carbon escaping.
as for the actual carbon you can buy it from some of the biggish koi outlets but for a better price try a home water purifier shoppy.. normally comes in 1kg 5kg or 10kg bags. its pretty light so 5kg goes a long way
this might be an idea you could use.
hubby says that if you don't want to fill the thing up completely with carbon then stuff some filter-wool at the bottom and top to take up the space. this will also stop the carbon coming out as well.. ( it might just work)