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This fellow on Ebay (located in Israel, but the shipping was darned fast) is
custom altering/making optical viewfinders at quite a few different focal lengths - for rangefinder cameras, of course. Basically he's masking off exact squares on wider finders - simple and inexpensive and I'd do it myself if I *had* a source for wide finders laying around
Mine came in today (I got an approximate aps-c 14mm finder for the middle of my 10-17 range) and it's quite nice. Nothing fancy, but good clear glass, defined, level - all I need to frame a shot in bright sun (and, of course, at this focal length, I don't even have to double check focus :P). The viewfinder aims just a bit high - I'll probably stick a sharpie dot to define the top of the frame, but all in all very usable.
His prices are crazy reasonable ($20 with only $11 shipping to the US). Definitely worth trying him out. (hilariously, he has a couple of finders marked 'tele-wide' for $53 bucks - I hope they're reeeally nice compared to the tele-wides for $20 :P)
The focal lengths he's offering are:
12
15
16
18
20
21
24
25
28
.