Stainless steel metric flat washers. Sold individually, choose size. Fender washers have a larger outside diameter. See below for actual sizes.
I.D. O.D.
M4: 4.3mm 8.8mm
M5: 5.4mm 9.9mm
M6: 6.5mm 11.8mm
M8: 8.5mm 15.8mm
M10: 10.6mm 19.7mm
M12: 13.1mm 23.8mm
M14: 15.2mm 27.8mm
M6 fender: 6.5mm 17.7mm
M8 Fender: 8.5mm 23.5mm
M10 Fender: 10.6mm 29.6mm
M12 X 30 Fender: 13.1mm 30mm
M12 X 36 Fender: 13.1mm 36mm