Another look at the brochure revealed that there was, indeed, going to be catering! What was more, there was even a small note at the end of the brochure that advertised the company that was being used: Hannibal's Catering and Events.
Furthermore, as the Magi discussed the possibilities for obtaining the Radio, they realized that the radio was likely already in the gallery. The building the gallery occupied was about twice as large as the showroom floor, and a door was set back in the far wall of the showroom, no doubt leading to a large storage area. Given that the building had once been a warehouse, it made sense that only half of the space had been converted into a gallery, with the rest kept to store pieces for future display.
My apologies for not making this more clear, I forgot that it hadn't been explicitly stated before. Half the building is gallery, the other half is storage, with an interior door connecting the two halves.