"shouldn't be to hard to find 358 2nd hand 757's"
Since most of the 1000+ built are still flying or flyable, it shouldn't be hard at all, at the rate the USAF could need them - even assuming a sizable quantity of KC-x equivalents are really needed. I saw some 757 parked at KVCV just yesterday, along with MD-11 and 747-400 cargo planes and scads of others. Aerial tankers get 10-20% of the utilization of airliners, so half-life airliners are bargains that will last longer than they'll be needed.
In the US, these are called pork barrel projects, where ranking CONgressmen get fat govt jobs for their constituents, in order to aid re-election. That's why Sen. Shelby of Alabama is so hot for the Airbus tanker, and other CONgress critters favor Boeing. If there were a CONgressional constituency for converting used airliners, that's what would happen.
GB