A local fastener specialist may be able to help, I've even purchased single fasteners over the counter – but it does depend on what they happen to stock. Otherwise search that well known auction site – quite common to find screws in packs of 5 at modest cost.
I think a lot of the stuff ex China is copied from, or aimed at the USA – hence UNC or UNF threads.
[edited to correct typing error]
Edited By David Jupp on 30/01/2017 16:16:41