Online dealer.
Here is the one I prefer:
http://www.yamahasportsplaza.com/pages/parts/viewbybrand/default.aspx
There are others. I used to use BikeBandit before they lived up to the "bandit" part of their name.
Go to the above website and enter the make, year, model of your bike. Then select the "transmission" diagram. The parts you need are #30 (nut, $4.95) and #29 (lock washer, $3.75). Expect shipping to take about a week.
If you are in a hurry you should call up a local Yamaha dealer and see if they have the part in local stock. You can give them the SKU number from the website to expedite their search.
Make sure that you bend that lock washer over so that it holds the nut in place!
Rod