M.2 Mount Screw size is a metric M2 - 2.5mm length
I have the exact same problem. I solved it. The screw size is an M2. That is a metric M2. The length can vary but I think it is close to 2.5mm. The post that it goes into is the problem. If you have a post then an M2 screw should work. You can probably get one at an ACE Hardware Store. You won't find it at a Home Depot or Lowes. They don't carry screws that small. If you can't find one locally or need the post / then go on to Amazon and search for "m.2 screw kit". If you get it locally make sure you have or get a jewelers Phillips head screwdriver as well.