Sometimes a diopter from another manufacturer can be adapted. In this case I got a Contax branded negative diopter lens that matches my perscription for a few dollars on e-bay. I attached it to the back of a Rollieflex with some silicone glue. It could be removed if I so desired.

In another case I found that the Nikon round diopters screw right into the back of the BESSA R camera viewfinder (note: the Nikon diopters are purposely marked 'wrong', see Nikon website for details).