It seems like most of the aftermarket cartridges are distributed by Lawrence Factor. This brand is sold through dozens of outlets many of which could be googled. If you are into this for the long haul you might consider repacking the cartridge shells. Again, the media are distributed from LF. A major retailer of the components would be Compressed Air Specialties or one of several others. This gets a little more complicated if your unit is engine powered. Standard practice is to pack a "triplex" package with charcoal, Vaporshell 13X and hopcalite. The latter item is pretty expensive, like a quart costs $100. You would also need polyester felt pads to separate the layers of stuff. It goes something like this: 30% charcoal, 15% hop, 40% desiccant, and the rest are pads. I would guess the cartridges are about $35-40 and average cost to repack might be 1/3 of that. Since a ctg contains desiccant it must be carefully protected from ambient air. Transfer the ctg to the filter quickly and seal off any outside air.