I went down to the hardware store, but I could not find anything similar to the part. Finial nuts are abundant, but that washer piece is very hard to find.
I figured out that I have a ready supply of domed metal pieces available from soda cans.
Step 1: Supplies and materials
1. Can opener
3. Drill w/ metal cutting bit
4. Metallic spray paint (that matches the color of your fixture)
5. Scissors or tin snips
6. You may need some protective gloves and glasses
1. Empty soda or beer can
2. Something to spray on (I use scrap plywood)
3. Random wood screw (really doesn't matter what size, but something over and inch)