You would not necessarily need the Voigtländer version of the old accessory shoe rangefinder. There are also old Leica versions, and several other lesser known makers of these. The ones I found to be fairly good quality, and easy to adjust are called Präzisa rangefinders. I have three of those, though only one is metric calibrated. It took me a while to find these on EBAY, but once I got a few I found them to be a great help for using my old folder cameras. I have seen these outside of EBAY, though at much higher prices; hopefully someone might have an idea other than EBAY for you. Even if you did try EBAY for these, they are not that easy to find at reasonable prices.
I have some really old Kodak rangefinders, but not for accessory shoe mounting. There are some unusual alternatives that sometimes show up at old camera stores. One issue is the silvering coming off the internal mirrors. Another thing to watch out for is fogged optics. Many of these are very tough to clean and even tougher to repair. Shop carefully, and I hope you find a good example.
Ciao!
Gordon