I'd second the recommendation on the Cosina/Voigtlander finders, and would mention another alternative; the 28/35 combo minifinder. You can see all the C/V finders on this page:

Scroll down the page for the 35mm and 28/35 combo mini. There are photos of the mini-combo on a IIIg that give you an idea of scale. It's very light, compact, and well made, but has only .5 magnification, i.e. everything appears 1/2 life sized through it. My only complaint about mine is that the rubber rear surround grommet likes to roll out when it rubs against the cordura in my camera bags. A drop or two of glue would cure that. Looks like it's gone up $20 since I got mine.