As the threads are very easy to wear-out, it is not unreasonable to have to replace them every time opening the cover.
The screws seem to be difficult to find. It may be possible to get them at an Optican. Other than that, they can be ordered from something like aliexpress. The screws are normally in lots of 100 to 1000 pieces. While the cost for each screw is low, Fedex cost many times the package cost. So it is good to have the specifications confirmed early, so China Air Mail can be used.
With dimensions and a supply settled, there may be unofficial "national distribution centers".
I have a de-threaded back-plate screw and have assumed some data below
Pitch: (0.3mm (coarse, 0.2 is fine but unusual)) (There seem to be 9 threads on 3mm, so 0.3mm seem reasonable)
Back Length: 3mm
SIM length: 2 mm
Head size: 2 mm
Holes are about .5mm longer than screws and 2.1mm diameter
Quality: "Soft" (not stainless/titanium, to decrease risk that housing threading is damaged) Nickelplated OK?
I found a couple of suppliers. Those with best fit seem to have 2.5mm head. It should be possible to adjust head and length...
Cost is $6 for 200 pieces, shipping with air-mail about the same.