The assembly manual for some equipment I have refers to "#10 .030 flat washers" and "#10 .060 flat washers".
I've looked on McMaster Carr, Fastenal etc and drew a blank.
Measuring w/ calipers:
OD is 0.3745 and 0.3770 (so nominally 3/8")
ID is 0.20" and 0.206" (#10 fits)
thickness is 0.033"...