I am sure you can get wheel nuts at the moped junkyard or moped warehouse, see the resources section.
Having said that, before you give up, try a bicycle shop and do not assume this is a metric thread. Many older bicycle fasteners used a standard "cycle thread" which is 26 tthreads per inch regardless of diameter. This is very close to a 1.0 mm pitch but different enough to screw up threads if you are wrong, so take the wheel with you. You can also try a local motorcycle shop if the bicycle shop fails.