You can find them in any any electronics store online or local.
also on ebay
it will probably have more poles or more positions.
The only alternative i can think of is one switch on/off and a second as a selector for 12v and break light.
What is the connection with the 12V, you have an output going to the power supply?
Alex
---------- Post added at 02:18 ---------- Previous post was at 02:07 ----------
ok, i think the 3 poles are the inputs and the one pole goes to your circuit,
so it is constant on, off, and light when brake is pushed.