While I was in St Louis to volunteer for DASH, I did some shoe-leather investigation in search of potential puzzlehunters. I had a lot of fun helping at DASH... but we only had two teams this year. Those folks had only found out about DASH because I happened to know Prof Bryan Clair, who passed along word to some SLU faculty and students. Us organizers had tried some other publicity things, but they hadn't borne fruit.
So I tried wandering around a couple of other St Louis school campuses in search of recreational math clubs and such. This isn't necessarily a smart way to go about finding potential puzzlehunters, but it's what I thought of.
MU-StL has a Math/CS department with some kind of Math Club. In my wanderings, I didn't see much sign of it or find someone to talk to. If I found the departmental office (which, depending on what sign you read is in room 303 or 311), then there was a lady in it who was on the phone the whole time I was wandering the halls. One room in their area had a sign out front labeled 'Math Club / "ACM"' Why was ACM in quotes? I don't know. So if I wanted to track down a math club in MU-StL, I'd start by calling up the math department on the phone. I dunno how useful it'd be, but it's at least as useful as wandering around lost on campus.
Wandering around lost the fifth floor of WUStL's Lapota hall (an engineering building) was more promising. I saw signs for events organized by ACM. And a big ACM banner hangs over a lounge area; so the ACM seems capable of organizing gatherings and such. Looking up the WUStL branch of the ACM on the internets, we can see they are sufficiently puzzle-y to have their own puzzle trail, in which I failed to solve so much as the first puzzle but let's not hold that against them. (Am I supposed to recognize an ASCII cow saying "phizzbuzz" from somewhere? I've heard of fizzbuzz. I know about a dogcow that says "moof". But that's as close as I get.)
I found the WUStL math department office and asked the nice student behind the desk if there was a local organization who was into "recreational math, puzzles, that sort of thing." He said that sounded like the Math Club, who was faculty-sponsored (or something) by this guy, Professor Chi.
After I told Prof. Clair of my investigations, he pointed out, "You could have just asked me; we St Louis math people talk to each other about stuff from time to time." So of course, that would be an important question for anyone you do manage to get interested: who else should I tell?