That's a very rare item. It took me over a year to find such a diopter for my Hexar RF, I'm glad I found it (at leicashop.com in vienna, Austria). Keep looking into their site; they also buy and sell Leica clones, copies and other M-bayonet cameras. It seems they are pretty good at finding the oddest camera parts every now and then...
Another alternative would be to take an existing Hexar diopter and have it analyzed by an optics technician. I know that even the standard ('0' value) diopter has an optical strength, as even the standard VF entry lens performs an optical function in the RF system. Thus you could have the lens measured and then specify a similar lens that has -2 dpt less strength, which would then have to be fitted into than existing threaded VF lens ring.