Gibson/Badalson sells them and I am sure that some of the others like Bill Alphin and the like sell them.
FWIW, you can also go to Home Depot and get a set of four studs that are threaded on both ends(5/16). They usually come NF on one end and NC on the other. They will work, you just have to cut a few threads off(too long). Dress the cut end on a grinder and put a dab of grey paint on the exposed cut end and they will look like the real Mc Coy.
Mike