The term to search for is "captive thumbscrew". The captive part being the unthreaded portion which allows the screw to rotate freely in the panel hole. These will tend to be longer than normal thumbscrews you account for the unthreaded part.

10 x Computer Case Thumbscrews (6-32 Thread) PC Computer Case Fastener Thumb Screws,for Cover/Power Supply/PCI Slots/Hard Drives https://a.co/d/hm14iKl

Contact Corsair, they may send you some for free.

Unless it will drive you completely mad that they're not a perfect match for the stock screws, I'd just throw them in your pocket, head to a hardware store (probably going to have better luck at an Ace or True Value than a Home Depot or Lowe's, not sure what the nearest equivalents are in other countries if you're not in the US) and go find something that matches. They'll usually have a little thread gauge or you can just try some nuts, and once you know what thread size you're looking for, go through the bins and drawers until you find something that works. Something like a socket head cap screw would probably work if you strike out on thumb screws and threaded knobs and such, or hell, just some sort of regular screw or bolt if you don't mind needing a screwdriver or wrench.

If you strike out in-store, search on Amazon or somewhere like Grainger or McMaster Carr for a fastener with the right thread and length. (EDIT: based on the assumption that it's a 6-32 thread, and some rough guesses on the length, this might be a close fit)

My guess is that the unthreaded part of the shaft is to give a little extra clearance for loose tolerances, so you have some wiggle room if the hole in the panel doesn't line perfectly up with the rest of the case. You may not really need that, but if you find you do, you can probably just file or grind down that portion of the threads pretty easily. Easiest way to do that is probably to grab a little needle file set from harbor freight, pop the screw into a drill chuck, and go to town until it looks right.

If absolutely all else fails, you can always see if any hobbyist machinists in your area or online will turn you up a matching part on a lathe, or I guess you could pay a real machine shop to make one but that's probably gonna be pricier.

If you live in Europe I could probably send you some.

Check in your couch.

"thumbscrew" is the search term you are looking for. 6-32 sounds right. https://a.co/d/9AvShgu

Watching this because I have the exact same PC case as you do and I'm down one screw. I've been thinking about contacting the manufacturer and trying to ask really nicely for a couple of screws. I know for example that for heat sinks they'll often ship extra parts for the mount at no charge.

Send an email to Corsair support and ask if you can have / buy some screws. They'll probably just send you some. Worst case they can for sure tell you the length you need.

These look like mine too, but my case has 2 versions: one with course threads and one fine threaded.

I've destroyed 1/3 the holes in my case mixing them up. At this point, I just want a pack of large ones so I can finish stripping everything and live with something simple but wrong.

EDIT: dual_sport_dork answered the question I didn't know to ask. Awesome. THX!