m.2 slot screw size is standardised the size is M2x3.5 (M2 is the thread size/pitch and 3.5 is thread length in mm).
I would say your options are:
- go to a local PC store, tell them your situation and ask if they don't have a screw to spare
- try writing to MSI in your country, maybe they'll be willing to send you one
- buy a box of generic laptop repair screws on amazon - what I did when I wanted to put a second m.2 in my laptop, but had no screw in the 2nd m.2 slot, now I'm set with tiny screws for life