Just kidding! This is the #1 question all writers are asked, and the real answer is—absolutely everywhere. Writers have a different set of radar than the rest of the world. At some level, we’re always scouting for story ideas. We overhear things (I’m a shameless eavesdropper), see things, read things, experience things, smell things, remember things, gather things and file them all away for future use. I have notebooks filled with interesting names, snippets of conversation, random images, and so on. It’s all grist for the mill.
(By the way, I actually did get an idea at Target once—in the men’s underwear section! It was right around Valentine’s Day, and there was a huge display of heart-covered boxer shorts. It occurred to me that a pair of those would likely be the most embarrassing thing in the world for a fifth grader to be caught wearing, and so I put them on one of the class bullies in Spy Mice: The Black Paw. You’ll have to read the story if you want to find out what happened to him…)