Hehe Finder, I know what you mean. It's super tricky. That's true about the handling of word-end "ー" in Katakana (typically adapted words from foreign languages). Typically, especially in recent years, not using the "ー" is becoming more common, especially for technical terms. I had to deal with this a lot when I was designing Expedia website for Japanese market. 😉
For "rangefinder" specifically, you should stick with レンジファインダー.
OK, getting back to the topic. As jonmanjiro mentioned, I have little hope of finding "Japanese RFF." The culture of online discussion forum in Japan is a bit different from the US or many of European countries. I'd try Flickr first as well. "2ch" is the huge discussion board portal so to speak, but it's a bit too deep in many ways. I believe there are many rangefinder camera specific threads/forums in there, but I doubt you'll find anything like RFF there.