i've blown 2 of these keys out recently. i bought a pack of the smallest ones napa auto parts had but they were too big. i ground them down a bit to an ok fit but couldn't get them just right as in a tight fit, ended up messing up my clutch shaft. good thing theyre cheap. i think i blew the keys out by dumping the clutch with a little throttle and not pedaling, kind of a habit from my motorcycles.
bikeberry.com has them, overpriced, but they got em...