The USAF scoured the world buying up 707s to strip them of their engines, pylons, tailplanes, undercarriages etc. There was also an article last week in AW&ST abut the work having to be done to try and stretch out the life of the KC-135 fleet to it's out of service date.
In short, forget about trying to find 3/4 good condition airframes and spares to last long enough to make it cost effective. Also one of the driving factors in Boeing changing to the KC-767.
If you want an equivalent sized militarized airframe I'd suggest talking to Boeing about a 767 (KC-767) or 737 (P-8A) frame or, as Beagle suggest, Airbus.
Though I am sure BWoS will have a great plan to buy some second hand 757s and refurbish them at a special low price, guaranteed not to rise Your Honour....