That's a good question and something I would have elaborated on if it wasn't for the 2'000 characters limit. I find these companies either by visiting websites I've been a happy client of (and who are in a niche I'm probably interested in) or by searching for technological keywords (HTML5, PHP, web agency, etc.) on Google with results limited to my country. In my case the companies I click with tend to use the (mainstream) technologies I want to work with. I wouldn't know how to do if I were in a more abstract field such as the ones you mention, sorry.