If you can't find a fiber washer, can you find an aluminum or copper washer? I know my local hardware store carries those, so I use a small copper sealing washer. If the ID is too small, you can always drill it out.
You can also make one by hammering flat a small piece of copper tubing, drilling a hole, then trimming into an annular shape. Heat the copper, then let it slowly cool. This will anneal the copper, making it soft when you install it, providing for a good seal.
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