Headsets, Inc in Amarillo, TX makes a good kit. The power supply is a box, which you can clip to anything - shoulder strap, seatbelt, or anywhere you want. I haven't used it in a helmet, but it's a drop-in for David Clark headsets, after you drill a hole in 1 dome for the power cord.
http://www.headsetsinc.com/ Model M-03 is claimed to fit all military helmets.