They are easy enough to make yourself.
The components are easy and cheap enough to buy and the wiring diagram has been posted on PPRUNE before.
You can pick up a UK NATO Jack or US Jack for about £15. The 3.5mm socket from Maplins (circa £1). Or raid an old headset and cut-off the jack (keep plenty of wire). A multi-meter will help you find which wire is which.
If you are recording a feed the solder up to the speaker side of things. If you're wanting to play an MP3 player into the intercom then solder up to the mic feed.
I wired mine up so we could play Ride of the Valkyries through the SkyShout speaker system
If you're looking to record intercom for a film shoot then the other trick is to get the filmcrew to place one of their mics between the earcups of one of your headsets and gaffer tape them together to block out external noise.
Simples!