Actually now that you mention it I bought some screws on Saturday at Home Depot and they were made in the US (Spax).
It's a bit of a bummer about Apple's computers. On my self-built desktops there are basically two types of screws that are interchangeable between any computer I've ever built (the normal ones and the ones that self-thread into plastic fan). If I add a PCI card and need an extra, the ones that came with a Pentium 4 motherboard will do just fine.
I kinda get the "we need this very specific screw" thing for laptops or phones where thinness matters, but the trashcan Mac Pro is an exercise in weird priorities.
It feels like Apple's design team has forgotten that standard parts exist even when they'd work fine, or even better than some crazy integrated solution. Custom screws for everything just because you can. PCIe GPUs? Who needs em!? Gotta fit in this cylinder.
It's a problem they've created for themselves, and if they wanted to stop having it the solution is "Stop doing that." Go buy a normal screw like everyone else.