Don't freak... Forget every one above. You should be able to get a steel washer that WIll work at a Car parts store (ie. O rielly , Pep boys.) ; If it's strong enough for car purposes It will work for your plane. (I've tried it) If they aren't nearby you can try Home Depot... Before you think their nuts won't work... look for steel nuts I'm positive they have them. As far as the size Just measure it in mm and then with the ruler you bring to either of the stores measure the inside of the nut. Worst case scenario you have to buy 2 50 cent nuts because you can't tell wich one it is.

Good luck
Electrolight

PS Try a car shop first (they are however more expensive ( it might be 60 cents a nut))