If you want to go silly cheap, 2 land line telephones plugged into each other with a 9v battery on the red wire will work. Replace the handsets with telephone headsets and off you go. Might even be able to wire it into the car's 12 volts so you don't have to worry about batteries.
Never did this in a car so... Be fun to try, though. A couple of retro rotary phones on the console.
Thanks for the replies. I found a Peltor FMT120 for $150, so I am going with that option. However, I have some old phones that I might try to make into a back up system...
Now I need to find the mic and headset for this amp. The best deal I found on the peltor version is $106, which seems a bit steep for a mic and speakers. Any suggestions?