For small hardware, metal pieces, etc., check:

  http://www.mcmaster.com/
  http://www.smallparts.com/
  http://www.reidtool.com/
  http://www7.thomasregister.com/olc/asapsource/
  http://www.grainger.com/

For instance, Small Parts Inc. lists 4-40 nylon hex nuts at $5.15/c
(white nylon).  McMaster-Carr has black nylon ones at $4.97/c.
(Although it shouldn't be a problem, the black nylon is UV stabilized
and in my opinion it lasts much longer.)

McMaster-Carr even has laminated wooden nuts, although they'd be too
large for this.  (And their printed catalog has very high "thud value":
it's almost 3" thick -- many hours of browsing!)

Carl Zwanzig