Mamiya make rectangular press fit diopters in 1 diopter increments. See this web page for the range:
They are available on ebay from time to time. If you are short-sighted (and need a negative value diopter), you might be better off going for a slightly lower value. For example, my right eye optical prescription is -1.5 spherical, and that leaves me with the choice of -1 or -2 for the diopter. The -1 is much better (for what it's worth!)
With the -1 diopter, focusing is very good in my case.