well presuming it has fallen off in the gearbox then it needs to be retrived, the gear peices themselves can be bought from VW i am pretty sure, however you will need to know which colour it is that you will need, i believe there are green, white, and red. Mine was a red on (89 valver)
Retrieving it i have heard people just run it for a while, then drain the box, and all the reminance of the old gear come out with that. Not the best idea i reckon personally, but i have heard it works.