eBay

FlightCom Aircraft Intercoms / Recorder / Extender Tested, Lot of 4.
I just found the Sig Intercom, added it with no price increase.

You get five items:

FlightCom:

Model IISX Intercom

Model IIGX Intercom

Model III Clearance Recorder/Intercom (did not test recorder function)

Model SR-4 Extender with cable (works with any of the above intercom/recorders)
Sigtronics:

Transcom II (not tested, just found it, it's a freebie if you buy the rest.)

(All can run on 9V batteries, or aircraft power, or any 12-32 volt DC external power supply for testing headphones, training, etc.)

Can be picked up at KHAO (Butler County, OH) if desired.

I've been on eBay for more than 25 years, this new account is just for surplus aircraft stuff.