It is likely they buy the entire assembly from their supplier. They probably don't have a single gear to sell even if they wanted to. Have you tried going to your neighborhood Home Depot and see if they have a gear that will work? Next option search the internet for that size and number of teeth. Final option, since it is plastic it might be possible to have it 3D printed. These printers are becoming more widespread at places like The UPS store, FEDEX stores and the like. Have no idea of the cost but it is probably worth investigating.