If you have one of these:
you can use the ones from the adapter, just unscrew them from the adapter.
 
The standoff you need is a standard one used on all graphics cards, adapters and cables.
 
The only difference between different ones would be the length of the threaded post of the standoff but the thread pitch and post diameter are all the same.

They are called a jack screw or standoff screw and for use with DB9, VGA or DVI you would be looking for a 4-40 machine thread and anything between 7-9mm should do.
 
I have quite a few of these in my hardware and parts bins that I have saved from equipment teardowns over the years.

4-40 is machine thread. M2, M3, ... is metric. Totally different things. Don't get "M2, M3-5" and expect it to fit; either the thread size will be wrong, or the number of threads per inch (or mm) will be wrong.

He gave a size already: "4-40". 4 is a machine thread size (diameter: not in inches or mm, but a specification) and 40 is the number of threads per inch. 1/4 to 3/8 inches is the length ("7-9mm" length). I don't know why length is measured in mm ("7-9mm" length), since machine screws are typically measured in inches; "40" is 40 threads per inch ... might as well measure length in inches as well.

"4-40 standoff" (or "jackscrew") with "1/4 inch to 3/8 inches long" (or "7 to 9 mm long") is all that you need to know.

For example:
https://www.amazon.com/4-...f-Screws/dp/B07PGW9XPN