This topic comes up often.
Basically, sticking a microphone up into the headset earcup does the job fine.
Obviously it needs to be a small one. A few years ago I bought such a mike from the USA, complete with a pre-amp, and this is used to feed a camcorder which has a manual audio level control. This is a "semi pro" kit and wasn't terribly cheap. I posted the full details of the stuff here back then - no idea how you would search for it now...
A different approach is to connect a camcorder's mike input direct into the intercom, via a fake headset plug. This also works, with some fiddling with attenuation resistors, and does away with having the mike stuck up one's headset. The one gotcha is that some audio (e.g. autopilot announciations) may not appear on the rear seat headsets, so if you are flying two-up already, you have nowhere to plug the thing in where you get all the audio.