Your currently limited to finding as others mentioned the old AMT/ERTL release, which hopefully Italeri will also release since they hold the molds to the KC-135.
Minicraft has a family of KC-135s coming out in 1:144 starting with a KC-135E which is a good starting point for your RC-135. Depending on which variant you're after...will depend on which mods you'll need. RCs came in a variety of flavors.
When will the KC-135E hit the shelves??? When Minicraft has ample cash on hand to pay for the tooling.