I get mine from a local auto engine rebuilder. They are the same as used in the older 2 litre Toyota Camry, 25mm diameter. They usually are happy to do a swap, for little or no money. My guy also has a shim grinder, so can shave one down in size, if necessary. Hope that helps you.
1982 CB750F
1978 Triumph Tiger 750
197? Yamaha DT175
197? Hodaka ACE 100