You might want to try Mcmaster Carr. The 6mm that I bought are slightly longer than the original screws so you may want order the 5mm.

91801A151
Metric 316 SS Flat Head Phil Machine Screw M3 Size, 6mm Length, .5mm Pitch, Packs of 100, $6.76

91801A150
Metric 316 SS Flat Head Phil Machine Screw M3 Size, 5mm Length, .5mm Pitch