Welcome to CS.
The washers are metal not plastic.
If the grinder is still under warranty ring Sunbeam and ask them for some.
If not, pull the grinder apart and find the existing washers, under the top burr, and go to a hardware store and buy some identical ones.
Youll need to add a washer or two to achieve a finer grind.
How long are your shots?
Single or double basket?