This should hopefully be an easy question.
I have a VESA arm, an acer 27" monitor, and no screws. I lost them somewhere.
Looking on Amazon, all I see are big bulk packs. All I need are 4 screws.
What do I need?
Buy the bulk pack and then become the Qt3 reseller of VESA screws for everyone else who inevitably loses theirs in the future!
Find some M4 screws at home depot or lowes.
Well, check the size first, might be different across manufacturers though I think M4 is most common?
Looking at the VESA standard, it appears that M4 x 10mm is the standard screw size for 100x100 mounts, which is generally what you see on monitors.
So something like this would probably do it. But double-check the size of the mounting pattern.
Have you considered drilling many small holes in the monitor itself to mount it?
I did. But there is a lack of screws at any rate.
Sorry, old forum joke (before my time, really):
Oh. I know.
I was here then :)
Pesky text failing to properly convey dry humor. 
I’ll just be over here blending into a wall in embarrassment.
Match what you have below to the type of screw you need:
| VESA Standard | Hole Pattern | Screw Size | Screen Size | Monitor Weight |
|---|---|---|---|---|
| FDMI MIS-B | 50 x 20 mm | M4 x 6 mm | 4-7.9 | ≤ 4.4 lbs |
| FDMI MIS-C | 75 x 35 mm | M4 x 8 mm | 8-11.9 | ≤ 10 lbs |
| FDMI MIS-D | 75 x 75 mm or 100 x 100 mm | M4 x 10 mm | 12-23.9 | ≤ 30.8 lbs |
| FDMI MIS-E | 200 x 100 mm | M4 x 10 mm | 24-30.9 | ≤ 50 lbs |
| FDMI MIS-F | 200 mm increments | M6 x 8 mm ; M6 x 10 mm; M8 x 15 mm | ≥ 31 | ≤ 250 lbs |
As a guess yours is 100mm x 100mm
Get the screws at a big box store where you can buy singles.
EDIT: Lol, I went to look at website prices and you can get whole kits of MANY for as cheap as you can get a few singles. That’s market pricing for ya.
For what it’s worth, the group I work for has to do monitor mounts from small all the way to 86" displays. We roll with a multi-pack, because you never know the depth you need depending on mounting arms or brackets, etc. Some of the packs include spacers as well. I don’t remember any of them being more than about $15 for the pack.
Something like this (being a smaller version):