It might not be much help, but you might try a mom & pop hardware store which carry a large selection of mini screws.
A long time back I was looking for a small nut to make a homemade spring-loaded top screw pin for a AGA-T Genesis
rta. I went to the popular big-time hardware stores like Lowe's and Home Depot whose selection sucked, but a local mom & pops hardware had just what I needed.