Looking on the CD I have installed on this PC for the steam driven cockpit it only shows the avionics master as a single switch which is 2 in one for both busbars connecting through a fuse to it, are you sure you are not after the battery relay? failing that I may have the newer disc at work and will have a look for you on that. I take it the TV version has a relay in it....
Where to buy from would be CSP at Oxford ( ex CSE aviation parts department), or the likes of Denis Robinson.
Incidentally one thing the Aircraft does have that is superb and that are the switches, they are a combo switch come circuit breaker which trips the switch off, so you do not need a separate CB panel... the other thing is the beacon, the base is about 1cm thick and contains an integeral built in flasher..