Hi Jeremy,
Is it the wee little screw that fits into the knob that you are after?
If it is, these would be called grub screws. By the look of things, the ones for the knob very likely will be M3 in size. If you have an M3 screw, try it to see if it will wind in. If it's the right size, it narrows your search.
Found this assortment of grub screws on Ebay UK:
https://www.ebay.co.uk/itm/ASSORTED-...AAAOSwLVZVnQa7
Alex.