There is a yahoo group
in STL. Not sure how active it is and you have to join to get info, but you might be able to find some poly-specific or poly-friendly events that way.
OkCupid is also a good poly-friendly site. If you make a profile you can search for "poly" as an interest and see if many profiles pop up.
Other than that... Just throw it out there now and then. Like LR said, a lot of people aren't really "out" until they get to know you or find out that you're poly too, so just being open and receptive will probably help. The LGBT community around here is almost anti-poly, but the poly community is almost as large so there is probably a bit of underlying competition that feeds the animosity. There is some overlap, but not much. It varies quite a bit by area, it seems, so trying to meet just one other local poly person is a good way to get started.