Actually, you first problem is going to be creating a proper environment to culture the bacteria. But more importantly is how you intend to acquire pure cultures to start with. When I buy bottled nitrifyers from Dr. Hovanec I know I am getting the proper bacteria and I am not getting anything harmful hitching along. One can't say this when taking media or gravel from somebody else's tank.
There area a number of companies that already do this. They are doing better and cheaper than you would be able to as well. But then they have laboratory grade equipment, use trained microbiologists and are generally experts in the field. They sell to the aquarium industry, to public aquariums, to aquaculture facilities and to waste and drinking water treatment plants.