Silver Plated Brass Screws for tuning
Buy and plate standard screws. However, when you make the cover (or whatever) with the threaded holes use an oversized tap to ensure a fit after cover plating (i.e. adjust the taps pitch diameter). This is how we did it in combline filter world. You can get almost any p.d. you want. I stripped a 2-56 hole and had it tapped up to 3-56 to save an expensive device. A good designer can figure out the appropriate p.d. depending on screw size and plating thickness. In contrast, screw manufacturers probably wouldn't make nonstandard screws unless you were willing to buy maybe 100,000 of them.