The dealership should have the washers, if not take the old ones to some of the other local auto parts stores... shouldn't be difficult to find a couple the right size.
In a pinch you can improve your odds or reliably using the old ones if you anneal them with a torch and then make sure the two faces are perfectly smooth by buffing with some 600 grit sandpaper... I don't
recommend this as I'm not accepting any liability if you do and you end up having to pull the damned thing back out because it leaks.