You can usually buy things like the switch ring on its own or complete with a standard Les Paul type 3 way toggle switch from any decent guitar store that keeps spares - I had to buy a switch as well which I'll use to replace one on another guitar so no problem with that for me. 

The nut was another matter, but it was from a DPDT push button footswitch of the type you find in older type stomp boxes.  These switches often come with a couple of thinner 'spacer' nuts which are designed to lock together on the switch threads so you can adjust how far the switch will stand proud of the pedal as well as a normal thicker nut and washer which fit on top.  Not all of these switches use the same size threads and nuts though. My thin nut was one of these 'spacer' nuts and it was just there unused in my tool box fortunately from some other project.  You do need to fill the recess with something on the front face to stop a thin switch ring from 'dishing' as you tighten the nut but you could probably do it with a washer or two under the switch ring which would mean that the switch isn't already held in place before you tighten the top nut over the switch ring, but with everything sandwiched it should all be tight enough once the top nut is on.  I prefer the thin nut approach better though.