What do you do when missing screws?

I've been messing with cars since 1963, and, I can honestly say that finding or getting the right size, pitch, head, and drive type of screws has never been an issue.
Between salvage yards, parts stores, hardware stores, friends, etc. It's just never been an issue.
Now it would seem to be even less of an issue with various fastener stores, the internet, "help" racks at parts stores, and. as someone has already mentioned, the "drawers" at the big box stores.