If you are talking about a cigarette lighter plug, you should be able to find one at any auto parts store. You might have to fashion some sort of mount for it, and I would make sure there is a fuse in the wire.
But then, if you want one with a cover to keep weather out, check on-line. I think that most on-line motorcycle parts vendors offer them, and mounts as well. I have a catalog from Aerostitch that has them. Items #4654, 4655, 4956 and 4957 at
http://www.aerostitch.com