same as all the other spark plugs.
go to ace hardware, lowes, menards, home depot, etc...find the socket wall, and look for the one marked "spark plug."
also, using a regular socket wrench on a moby as opposed to a special moped spark plug tool sometimes requires that you sit facing the front of the engine and use your foot to push the engine back to get the socket on and off. not too big of a deal.