When I needed shims for my 1982 CB750FZ I tried my local Auto Engine Rebuilding shop to see if 25mm diameter shims were common. They had hundreds, as they were used in the Toyota Camry 2 litre engine from about 1989-1993. They also had a machine to micro-grind shims to give me some "odd" sizes, as I believe the in-between sizes were only used in the factory, and not available as a spare part.
I hope this information is of some help.
1982 CB750F
1978 Triumph Tiger 750
197? Yamaha DT175
197? Hodaka ACE 100