The Univeristy of Minnesota has what you are looking for scatterd around campus. I don't have any pictures though there are several different styles that I've seen, some are round things about 2 feet in diameter, some are just bulliton boards mounted out side, about 4'x8'. Most common is just a large buliton board inside each building. In fact I'll bet that local grocery stores near you either have or would put up a buliton board for the purpose.

One warning, most people advertising an event cover the entire baord. (at least here) In the process everything else is obscured. I never really liked seeing 30 copies of whatever flyer, but they were always there. I don't know what can be done about it, but if you can think of anything, please do.