My motherboard (micro atx) lines up with these holes in a case I found. (pic related)
The black circles are pre-installed in the case (I can't remove those standoffs) and the motherboard lines up on them. The red circles are those holes (4 in total) that the motherboard also lines up, but there is nothing there for the screws to go in. So, I was wondering, what do I put in there? Are there any special standoff screws for these holes?
You'll need the adjustable stand-offs that would have originally come with the chassis (and may still be stuck to the motherboard last installed in it).
The odds are they were plastic slide-in ones that have snap-downs to lock the board to it.
Similar to these:
If you did not get them with the chassis/system when you purchased it, and you can't find proper ones via Amazon or alike (check the existing hard-mounted stand-offs' height), then contact the place of purchase, or the chassis manufacturer directly, to try and obtain some.