Boat builders use a lot of screws.  Try West Marine:

    http://www.westmarine.com/

You will probably have to settle for stainless steel, but they sell in
both small packs and bags of 100, and ship almost world-wide.

My other source of serious quantities of screws is Jamestown Distrib-
utors:

    http://www.jamestowndistributors.com

They have stainless, brass, and silicon bronze, but only by the box.

Wallace Venable