There are vendors on eBay that sell Yamaha keys by the code number (search Yamaha key). Remove the nut holding the ignition switch so you can swing the switch out and read the code that is stamped into it.
Unfortunately ordering keys by the key # from Yamaha is very unreliable as the seldom work.
The only fix I am aware of is new ignition cyl which comes with 2 new keys.
You simply unbolt the old igniton cylinder, and plug in the new one.
I have included a link to all the fast moving YT-6800 parts for you.
Simply scroll down through the list and select the link for "Main Ignition switch w/keys"