Okay, a quick web search shows that Microfasteners stocks M3x0.6mm
socket-head capscrews. The same thread also seems to be used in
Zebra barcode printers and is available as a replacement part
(HW49152) at replacement-part prices.
If it were just onesy-twosy, I suspect a shop specializing in old
French bikes and motorcycles (e.g. Motobecane) might have something
like that--maybe.
--Chuck