I can't specifically answer your question, but I've monitored two Costcos, one in CA and one in AZ. Both seem to favor Itinerent frequencies in the 151Mhx and 154Mhz ranges. Here's what I found them using, but I would suggest you sit in the parking lot and search 151 and 154.
151.625
151.685
154.570
468.2125
<>< Mike