Hi,
Any decent-sized hardware store should have 4 mm metric flat washers. Last time I bought them, I paid $3.50 per 100 at my local True Value Hardware. I had a few NOS Martano washers, and except for the rust on the Martano washers, they were as close to identical as I could determine with my plastic vernier caliper.
Hope this helps.
Earle Young Madison, Wisconsin, USA Offering expert wheelbuilding service for classic and modern bicycles.