you should be able to order some from the place you get the wheels from. just tell them you have a hub diameter of 64.1mm. most wheels will use 73.1 so i assume but you should check with whos selling you wheels. if you they dont sell you them, atleast find out, then you can get something like this:
Hub ring aluminum 73.1/64.1 [AN731-641] - $0.00 : Bimecc USA, Catalog