Where are the Western Electric 124 for sale in the USA?

I see some on eBay but they appear to be Asian sellers.
You should contact Deja Vu Audio (dejavu-audio.com).  They recondition/re-build 124s, 133s and other vintage amps, and make modified versions of such, utilizing a lot of vintage parts.  These are custom amps that can be made with different gain structures and different parts, per your specification (e.g., "cheaper Langevin input transformers in lieu of WE 618 transformers), etc.  

I own a modern re-build of a 133a amp.  It has WE input and output transformers, but, it utilizes a modern power transformer.  I run 348s and 349 tube in it.  My linestage is also a modern version of old Western gear (Western input and output transformers and other vintage parts), but, it also has a remotely controlled motorized volume pot.  Both the linestage and the 133 clone were made in Italy by the builder used by Deja Vu Audio.  

Deja Vu Audio is a retail establishment that also builds a lot of gear, much of it utilizing vintage components (a lot of speakers utilizing Western Electric drivers, IPC drivers,  RCA drivers, etc.).  They even have systems built around the Western Electric 15a horn (555 drivers) and a 16a horn (permanent magnet 555 driver--biggest horn I have seen).  

The info you provided is a big help.  I have a WE124c that has not been reconditioned yet.  I haven't decided whether to recondition it myself or sell it.  

Whichever way you go, you have a very valuable amp.  When properly reconditioned, it is one of the best sounding amps you can buy, provided you don't need much power.  I spite of being mostly used in commercial setups, hence heavily used, the basic parts of Western Electric gear are so well built that they last a very long time.  I recently heard a 124 that only needed minor parts replacement that sounded terrific.  

Also, Western Electric ran its gear so gently that tubes last a very long time.  My 133a amp, which has been rebuilt to run at even lower power than a regular 133a, has not required any new tubes after many years of operation (a good thing too, since all eight signal tubes are very expensive).  The 124 is a similar amp (slightly more powerful).  I have heard the 124 run with both 6L6 and the 350 output tubes.  I like the 350 more (larger, more enveloping soundstage and a bit more weight to the sound), but, it works quite well with the cheaper tube.  The input tube can also be the MUCH cheaper equivalent of the 348A (6J7 is the cheaper equivalent, I believe) without much penalty in performance.  The other thing to note is that, because it has an input transformer, it really works better with a linestage that also has a matching output transformer.  

One more thing to consider in a rebuild.  In my area anyway, power delivered to the home often comes in at quite a bit higher voltage than 115 volts.  I have seen it as high as 127 volts and I know of people who have measured 130.  I have not had problems associated with the high voltage with my Western gear, but, I know that my local dealer has had some problems with rectifiers in Audionote gear because of the extra high voltage.  There have been other brands of gear that have had problems with US voltages being higher than expected.   It might be something to consider in your reconditioning.