VanDykes Restorer's catalog lists some plain steel flat head screws.
They are in #4, #5, & #6 sizes and in 3/8" to 1" lengths and come in
boxes of 100.  I've never ordered them so I can't tell you about the
quality.  They also have them in plain steel round head.  Their online
site is  http://www.vandykes.com/ 

Jim Quashnock