"Old Style" cut thread wood screws have indeed become rare, at least
in local retail outlets.  Responding to piano technicians' demand for
round head screws for piano action flanges, I have sourced and stocked
several sizes which I have made available through my supply business,
Piano Forte Supply.  These screws come from Germany, where "old style"
screws are still available from a few sources.  See:

  http://www.pianofortesupply.com/parts/metal-parts/screws/screws-2/ 

If there is sufficient interest and demand for other types of wood
screws (flat head, oval head, or different sizes) I would be willing
to see if I can bring some in.  You can contact me thorough my website.

It is always nice to use the proper screws for the job, replacing
original screws with identical ones when the need arises.
Phillips-head screws, drywall screws or sheet metal screws have no
business in vintage pianos.

 [ Assortment of round head screws for piano work.
 [ http://www.mmdigest.com/Attachments/17/01/08/170108_221942_Action%20Flange%20Screws%2004%20B.jpg 

Jurgen Goering - Piano Forte Supply
Nanaimo, British Columbia, Canada
http://www.pianofortesupply.com/