The last time I needed mandolin rail clips I bought them at Player Piano
Company.  I don't remember what they cost, but there were 100 in a box.
Looking at the old box I have, they are PPC# 848.  Hopefully PPC still
has them in stock.

The one and only internet source I could find is:
    
    http://www.advantus.com/store/detail.asp?pc=05&child_cat=13&group_id=24

$6.00 a box of 100.  You'll want the "2PETITE" kind.

Call Player Piano Company first though.  Make sure we support our almost
only vendor of player piano supplies left.

Ray Finch