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Failing all of the other good advice above:
McMaster-Caarr at http://www.mcmaster.com/
Brass Slotted Pan Head screws - part # 90928A211
(M2.5 x 10 mm long) - .45mm pitch) $12.33 per 50.
Or 316 Stainless (non magnetic) - part # 92290A060
(M2.5 x 10 mm long) - .45mm pitch) $14.22 for 50
Brass washers - part # 92916A315
(.101 i.d.) 100 for $1.89
Pricey, but if you have a few friends, you can save some for stocking stuffers.
Note that 10mm can be slightly long for some combinations of cartridges (with bottoming holes) and thin headshells.
8mm might be a safer all-around bet. Measure your single good screw and look up a different part #.
Thom @ Galibier