The 105mm lens is the equivalent of something like a 45mm lens in the 35mm format. It doesn't seem to be so easy to find a viewfinder in the normal range. Voigtlaender used to sell one but doesn't any longer, and I bet it costs a pretty penny if you find one. There's a Soviet turret-style universal finder that seems to run about US$100. Your best bet might be to try using the stock viewfinder for a while, and monitor eBay for a chance to get a 50mm viewfinder at a lowball price.
I've never used the 6x9 version, but I've found the finders on 6x6 Nettars to be usable as long as they're clean. They're small but not ridiculous.