i bought a bag of 12 nos headbadge screws off of ebay awhile back,just for inventory. there is a store on there called retrobicyclesupply,that carries alot of nos small parts. check them out!
go to the source, look for places that deal with screws /fasteners in the yellow pages.
here in london i hit facco fasteners. never ran across a screw/thread they couldn't replace.
even those exclusive threads for sturmey hubs and old guns.