I believe they are all a standard size from what I have read. M.2 3mm deep.
Here is one sold on Amazon, and most of the negative reviews are about the price the seller is charging. I would never pay that much, but at the same time I wouldn't cry about it after I bought one.
https://www.amazon.com/GutsParker-M...364483&sr=1-1&keywords=m.2+standoff+and+screw
Did you lose yours from your motherboard? If not, and it was missing, you might be able to get the manufacturer to mail you one.
Edit:
I just checked Ebay, and some people are trying to charge $45+ for one M.2 screw and standoff.