The tire must be removed to replace a spoke. Unless your dealer is willing to break open a pack, Yamaha does not sell one spoke. You must buy the whole set.
It is not a recommended practice, but I have ridden for extended periods with a missing spoke. You do have to completely remove the broken one, and the rest of them better be adjusted correctly. Just remember, the spokes make up the structural integrity of the wheel. A missing spoke puts more load on those around it.
I had the same thing happen to me. Stock Honda replacement kit was expensive so the best deal I was able to find was a "heavy duty" spoke kit that doesn't require any drilling on the hub. it's made by Rad manufacturing