Name of part - Rear wheel spacer screw
Dropout adjustment screw.  Here's a thread from the Bikeforums with some links and such to check:

http://www.bikeforums.net/archive/index.php/t-274077.html

Unfortunately they do not appear to be standard... You might try haunting more established shops to look through their old-parts boxes.
Most hardware stores carry a very wide variety of metric fasteners; you might be able to find a suitable screw there