Look at the home page for this site and you will see a button for aftermarket parts - this lists all aftermarket part manufacturers; There are 7 or 8 different manufacturers of countersprockets and rear sprockets for the bike (all the 2t and 4t models use the same sprockets). The countersprocket is unique to gasgas, whereas the rear sprocket is the same as a late model husky.
For the listing of sprocket manufacturers, look here :