And my answer (which, of course, is specific to me. Other writers might do things differently.): Ideas for novels come from everywhere.
Until your imagination kicks into high gear and gives you more ideas than you can ever want, you might try glancing through newspapers, magazines, online stories, etc for ‘story starters.’
You don’t have to get the whole story because that’s not what you’re looking for. You’re just looking for something — a word, a phrase, a description, anything — that will click in your mind, and then you’ll be off and running with a story that unfolds easily and fully. And chances are that it won’t resemble that ‘story starter’ at all!
When your imagination reaches the point that you don’t need story starters because your mind is deluging you with ideas, then you simply listen to what it’s giving you and weed out those that aren’t right for you — or the genre you are writing in — and you hone those that are right until you have the shape and fill of a story.
Then you put the seat of your pants to the seat of your chair, stare at your computer (or whatever you use) and get started.