Radio Shack has all kinds of adapters like that. If they don't have it in the store they could probably order it. If they can't order it, they should have the parts for you to make it yourself; it wouldn't be that hard.
You're going to find that you have to turn up the volume very loud to hear it-there is an impedance mismatch there, and it will take more to drive a headset. I do it anyway with mine, works great, just remember to lower the volume before you unplug the headset.
You will find that a regular convertor from Radio Shack may, but most likely, may not work with the intercomm as the impedance values are different. I assume your goal is to record the cockpit conversation. You may be able to search on google and find a schematic on modifying radio shack parts to match the impedance.
Alternatively, if you have one of those David Clark-like headsets with a smooth cup, you can buy a telephone suction mic from Radio shack and just attack it to your headphone. It works pretty well and is very cheap. (under $5)