"So how is a potential player who is not "in the know" find out about these places. Do you go up to a complete stranger on the street and ask about any cardrooms?"

Actually the way I found the underground cardrooms where I used to live was by talking about poker with people. "So what did you do this weekend" "Well I drove three hours to the casino to play some poker"

Eventually that lead me to meet other people who played poker. This got me an invite to a home game, someone at the home game invited me to a game at a "social club" and from that game I heard of of ther games etc.,etc.