Look at the crankcase segment in the fiche. There are four plugs, one for each cylinder, all identical, cost $4 each from Yamaha Sports Plaza:
part is 90338-11152-00
These plugs usually start weeping after a couple of years. I've made it a point to get new ones whenever I change out the coolant. However, I've never seen one do more than weep a few drops at a time, so it is something that can be lived with eternally, if necessary.