I've never found any 3mm woodruff keys at any hardware store, small engine repair, boat repair, motorcycle, automotive, etc. I did however use an 1/8" one and a file. It takes a while, pretty hard metal, but it works. As far as it working for long, I wouldn't bet a high dollar motor on a cheap hardware store woodruff key as much as I'd trust Viagra from a guy in the alley. Get the Kawi one for long term use.