If you can't find a suitable piece locally, there's some good online sources for small quantities of stainless stock:
http://www.onlinemetals.com/merchant...ep=2&top_cat=1
https://www.mcmaster.com/#standard-s...heets/=1btwvvh
https://www.boltdepot.com/Washers.aspx?nv=l
Email and they can tell you the thicknesses of the washers. Most front sights are about 1/10 inch thick. Measure the old sight.