There are many different thread types used throughout the world. However, most screws used in the developed countries in "the west" in the last 25 years conform to "standards" that are either international standards (the metric sizes), or former British or American "standards". Metric sizes should be readily available. The self tapping screws are available in the UK, but I am not sure about elsewhere.
Anyway here are the details of some of the screw used on one of my machines.Atari 520 STFMCase screws
Threaded section 9.6mm/0.38in, total length 12mm/0.47in, thread outside dia 2.9mm/0.11in.
The nearest type that I have found are "self tapping screws", size "No.4" length 0.5in (so it says on the packet, but when measured is approx 0.54in), but these are slightly too long so either cut or file them down, or use a M3 size washer under the head of the screw.PSU mounting screws
These hold the PCB to the bottom plastic case, and also the ones that hold the PSU to the bottom plastic case through the PCB):-
Threaded section 7.7mm/0.3in, total length 9.8mm/0.38in, thread outside dia 3mm/0.11in.
Again use "self tapping screws", size "No.4". If you can find ones that are the right length, or cut down some of the 0.5in ones.Metalwork screws
Machine screws that hold the metalwork covers (such as the metalwork over the PSU) to the main PCB metalwork:-
Metric type M3, threaded section 4.7mm, total length 7mm, thread outside dia 2.9mm.
There are some other odd sizes, but I did not need to replace these so I did not measure them.
Hope this helps
Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (nearly forgot the Psion's)