you can try McMaster.com, they sell pretty much everything that's available. Not sure if this is a unique fitting or not, but their site is pretty easy to navigate.
If u order, or buy (local pick up) the best is http://www.travers.com/ (Taps/Dies, Drills etc.)
u can buy Madison Switch also from http://www.grainger.com
if u got the Tap ( i got one , i work on Park Av. and 59 st. if u still need it ?) no need nuts, or u can buy from e-bay, just for nuts, the switches r garbage. Grainger got more then one kind of Madison Switchs.