If it's the acorn nut that holds the muffler bracket to the passenger foot peg, then it is a M12-1.25 nut, which is fine thread, and which is the holy grail of acorn nuts. In other words, if anyone has a source for M12-1.25 acorn nuts, I'd like to know about it.
In my experience, you will be doing good just to find a regular M12-1.25 nut locally.
I have some stainless (non-acorn) ones that I got from McMaster-Carr, if you cant find one locally I'll mail one to you for a small fee.