Have a look on the David Clark website (
David Clark Company Incorporated) under aviation accessories (the hotlink is over on the left hand side of the page) and see if they have what you want.
Something to bear in mind though, unless your own headset is a noise attenuating model then you are going to struggle to hear and be heard over the interphone. The background noise on the ramp can make communication difficult to say the least.
If you are using your own headset as opposed to the company issue there may also be legal plus health and safety issues to consider.