Only on some third-party sites: I did not see Frontier flights on expedia.ca, which is where the OP initially searched

But if the OP has no problem with flying Allegiant -- taking into account that Allegiant will not rebook on other carriers in the event of IROPS -- there's no reason not to fly out of IAG. IAG is not the only airport where Allegiant is the sole carrier. One can plan around the lack of terminal amenities.