Many authors who get paid for their work keep notebooks they fill with their ideas. Similar to a traditional journal, as ideas occur to them, they see or hear interesting things throughout their days, or they find snippets of material as they're reading other work, they record it in a notebook. Sometimes it's a traditional bound paper book, but technology has allowed writers to do this digitally, too. Some utilize a smartphone's camera, and others use apps like Evernote to archive material as they find it.
The point, though, is to be open to topic possibilities all around you and capture them in real time to refer to later.