Why not make your own?
You can also use activated carbon foam mats as they are offered in aquaristic trade, and then cut them into cubes. Mix these cubes with rock wool flakes, and your selfmade gpito is done. (5$/m² e3ay, take this with big pores, in 1" thick and cut about 1" big dices)
On the activated carbon surface, beneficial bacteria and fungi can settle, same principle with bio-carbon in your compost. In the aquarium the bacteria settle on the carbon surface and decompose nitrates (fish shit) and as a substrate they give room to trichoderma, mycos and useful bacteria to colonize.
That's not rocket science, they were just the first to come up with the idea of mixing both.