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You might have the best luck going to one of the many fastener suppliers. One of them will probably be able to duplicate the missing machine screw, whether it has an imperial or metric thread. Here's just one of many such companies:
You're probably looking for a slotted pan head machine screw. If the aesthetics are important, it might be more difficult to duplicate the milled circumference.
With luck you'll find the right one and not have to buy 1000 to get one!