This is the pinout for the driver's headset on an Interstate

Radio shack has the 5 pin Din connector, and some pretty nice headsets. They also have headsets with mics that will work on this system, I've found a nice Koss headset that works well for $15 at Radio Shack. Be careful though, not all mics have the proper impedance to work correctly.

This is the plug on the bike, as you'd be holding it and looking at the end.