Say what !

Look around your area for a specialty hardware store.
Here in SoCal we have McFadden-Dale Hardware (mcfaddendalehardware.com) with 4 locations in SoCal and one in Las Vegas. They carry fasteners I didn't even know existed, and lots of other stuff.
Hillco Fasteners (hillcofasteners.com) is also an outstanding source for specialty hardware.
And don't forget sources like McMaster-Carr (mcmaster.com). They have locations in Atlanta, Chicago, Cleveland, Los Angeles and New Jersey.