Googling the world's greatest information source, I find that an Italian
company called Clipiola makes a round paper clip which is about the
diameter of a penny.  Tins of 100 sell for around $8; multiple U.S.
outlets are listed.

Is this the approximate size that's needed for mandolin rail clips?
That is not an academic question.  I anticipate the need for some for
this application myself in the near future.

Dan Harrett