I only knew of their chopper located at Mercy Jefferson. Last I heard Air Evac was in Hannibal. I even had the chance to see their base (mobile home) a couple years ago. I guess they are expanding. Try the main ARCH freq which I think also goes by Air Comm or something like it, for the St Louis area. ARCH used to have UHF freqs all over MO & IL but not sure what all they do now. I know it's not much but I hope it helps.
AirComm is the dispatch center that operates (assuming most) of the Air Methods communications systems. ARCH has multiple UHF repeaters dispatched by AirComm. They may or may not dispatch for other agencies.
AirEvac's nationwide communication system is based out of Ofallon MO. The operate on multiple VHF frequencies, many of them 158.4 and 151.505.