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.
Doing a CloseCall is the best way since they're likely not using and officially licensed frequencies. Being in the parking lot may be too far away though. Wandering through the store should work. I'll just put my SDS-100 on "Search with Scan" and CloseCall on and have it record. That way I don't have to be constantly listening to the radio in the store. I just let it do its thing and then play it back later. I find numerous business frequencies that way.