Regarding new screws, I find American Piano Supply has a large stock, but
their screws are not as good quality as the originals; ie, threads and
slots are cut very shallow.  I once sent them an old screw next to one of
their's so they could see the difference (no reply).  Of course, good
screws are available, if you don't mind Torx, square-drive, or Phillips.
I can't use these on restorations, though.  It would be nice to know if
any firms in the US or Europe still make good-quality slotted screws.

Philip Jamison