If you can't find the real thing (I have never seen one in the flesh), this
article includes the Hallicrafters service bulletin describing how to build
an S-meter for the S-40, et. al., along with other info about building your
own.

http://antiqueradio.org/smeter.htm

Alternatively, this article describes how you can build a "magic eye" meter
for a Hallicrafters receiver (in this case, the S-20R).

http://antiqueradio.org/magiceye.htm

Have fun!

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html