The answer is everywhere. A word or phrase can fire a writer's imagination. Overhearing a conversation in a cafe, or the way a person cocks their head, or watching strangers say goodbye at the airport may spark a story. News items, children's antics, cloud formations, an unusual name or the scent of a nightblooming flower, can be the match to burst a twig-size idea into forest fire.
Writers can discover things to write about anywhere they care to look.
Frequently a character picked from real life or imagination demands to be written about, maybe because of a special strength or flaw. A quality which makes that character react in a special way to problems and situations. One story of mine actually developed from a used typewriter ribbon, one from a pretty name, another from a missing housewife and her cup of coffee. An intrigue can blossom secret passages within the walls of a fourteenth century German castle or a curious classified ad. The stealthy silhouette of a man standing by a street lamp on a rainy night can inspire a mystery.