Everywhere and nowhere. From songs, commercials, snippets of conversation, a news story, literally anything can trigger a story. Sometimes the story grows and ends up not being anything like the original seed. Sometimes it just gets filed in my idea document and molders there. There is a great T-shirt I’ve seen being sold by writing chapters:
BEWARE OF WHAT YOU SAY. IT MIGHT BE USED IN MY BOOK.
That about sums it up.