Desktop motherboards that support M.2 ssd's usually come with an M.2 ssd screw and M.2 ssd standoff. Portable pc's usually do not come with the screw and stand off unless the mobile pc already came with an M.2 ssd. Both items are readily available from online vendors. Thanks to an international standard there is only one universal size for the screw and standoff.
The correct screw size is 2.0 x 3mm (CM2x3-3.3).
If you are in the USA, then you can usually find the screws at Ace Hardware stores. Make sure you check the length. Some of the screws at hardware stores are too long.
I've also seen them available at Amazon. They were advertised as replacement screws for M.2 ssd's. Mod DIY has them too:
Just do a search for: M.2 ssd screw...