I have two Japanese cars ( a Mazda and a Nissan ) which use cams placed directly over the valves ( no rocker arm ) which contact a shim sitting on top of a bucket over the valve. Are the shims used by different manufacturers all the same circumference such that I could use for example Toyota shims in a Nissan or Mazda or visa versa, or are the all different circumferences? I.e. are these type of shims standardized?
Almost never will you find two sets of shims from different manufacturers that are identical. That said, it's a shim. You can reshape a larger one if need be, though it's a hack job.
So unless you really want to go DIY, no, they're not interchangeable even if they're the right thickness.