McMaster-Carr Supply Co, http://www.mcmaster.com/ has most kinds of
hardware.  Their slotted oval head screws are limited to stainless
steel and brass.

Zinc plated hardware is easily stripped with muriatic acid.  It only
takes a few seconds.  Once the parts are stripped, plain steel can be
made blue by heating to about 568 degrees F.  Heat the parts in Pyrex
or ceramic, and quench in ordinary motor oil when done to arrest the
coloring process.

John Nolte