Get in touch with Headset Services Limited at Shoreham, a google search will get you there. They have all sorts of adapters, and a very good mail order service over the phone. As a rule Boeings don't have a power supply for headsets (although I have only been flying the classic 737 rather than the NG), but I am sure that HSL can guide you to the right adapter. The adapters aren't cheap, but we all know the value of a noise cancelling headset!