http://www.burgess-switch.com/mediando/index.php?IdTreeGroup=41&IdProduct=99
http://www.burgess-switch.com/mediando/index.php?IdTreeGroup=41&IdProduct=213
Interesting find! Have you found out anything regarding prices etc? I'm still after one for my horn.
There are lots of switches that can fit. It is the metal bracket it affixes to that is the hard bit to source!
You could try and make a simple U shape bracket out of thin sheet metal and clamp it in between the standard bracket and the handlebar.
If you give it an H shape with a thicker horizontal bar wide enough to go underneath the clamp, then bend up the sides and drill small holes for the switch. (metric 2mm / M2 will fit nicely)
Maybe a project for Chas ;)