Something which other folks haven't mentioned is that it's generally best to match viewfinder to the intended camera, especially for longer lenses; for instance, the Soviet clone of the turret is a good cheap option, but it's really intended for the Kiev/Contax system, and won't be accurate at all on a IIIf due to the difference in accessory shoe position. It's less important at the super-wide end - say 28mm and wider - but at 50mm and up it's very noticeable.
This caught me out when I was trying to save some cash a decade ago, and I don't know why more people don't point it out.
The Leitz multi-finders are obviously designed for the Barnacks - and so are considerably more accurate than most on a IIIf - but they're cropping finders. I don't mind the Imarect/VIOOH for 35mm, and it's tolerable at 50mm (although I'd rather just use the built-in finder!), but past that it's like looking down a tiny tunnel. You can generally find them fairly cheaply if you look around - MW Classic in London currently has one for £59, and Red Dot Cameras have a bunch for around the £50 mark, for instance - and it's not a bad bet if you're looking for 35mm or 50mm finders.
Definitely get a dedicated finder if you want anything longer, though!