Because my high school group cycled GMs session to session, we usually had several people comfortable with costing items and with running NPCs; so multiple characters could be run simultaneously through minor pre-game activities, like shopping; and it was our custom that when a PC went into a shop, they could ask the shopkeep if they had anything interesting, and whoever was running the NPC would try to come up with something new.
I got two books from a discount bin - As Seen On TV by Lou Harry & Sam Stall (2002), and 283 Useful Ideas from Japan by Leonard Koren (1988) - and if I don't use them for random town stuff I'll never use them for anything.