In my limited experience of aftermarket gear knobs they usually come with a couple of plastic sleeves and grub screws to hold everything together.
Why not just buy another 8-ball and work out how to fit it? You could easily epoxy a lock nut in like Greeni has, or use a bit of plastic tube to reduce the size of the hole. The thread on the transfer box lever should be able to cut a thread in plastic that will be good enough to hold it together.