| 3/16" is the correct size. You can buy one from a company called Bondhus. I believe that they make them for Fender. The metric 5MM one I bought from them looks in every way identical to the Fender branded one that came with my 2002 MIA Jazz Bass (except for the hex size, of course). As you know, you want to get a 'ball' end hex, not a straight hex. | 
