There are, at present, two centers for Pokemon in Central Illinois.
There's one centered around Champaign-Urbana. Specifically, the Gaming Goat. There's a Facebook page around that called C/R/U Pokemon (or something similar), and they're pretty active.
There's a second one centered around Springfield. Specifically, Yeti Gaming. They're also on Facebook, though be sure to find the one for Springfield and not the one in St. Louis. That one is run by the PTO that runs StL Regionals.