The Revell/Monogram B-52D seems to out of production right now. I haven't found any on any online hobby shops. They can be found on ebay but prices have been on the rise.
AMT did a B-52G early, B-52G late and a B-52H. They have sold the molds to Italeri and they are selling them under their name now along with the KC-135 series which were also AMT molds.
Meteor Productions is in the process of making several conversion kits and decals for the Revell/Monogram B-52D. They will not be cheap.
http://meteorprod.com/B-52Updatepublic080117.html
A company called Golden Dragon Productions makes some stuff but I haven't seen any of their stuff in person but have heard a lot of complaints of poor quality.
True Details makes wheel sets for the KC-135 & B-52. They also make a galley set for the KC-135. Airwaves make a KC-135 photo etched detail set.
KC-135A/E decals
http://www.bare-metal.com/Experts-Choic ... ecals.html
KC-135R decals
http://www.arcforums.com/forums/air/ind ... 138486&hl=
B-52 decals
http://www.arcforums.com/forums/air/ind ... 141487&hl=
If you do a search on the ARC fourms I linked above on B-52 & KC-135 in the jet modeling section you should find a lot of build information on those kits.
Mike