This question comes up over and over again. there have been countless responses. My favorite has to be Neil Gaiman's tree adventure.
For some reason there's a very real worry about where ideas come from especially from writers just starting out. I think this question stems out of fear. When you're just starting out it's easy to think all of your ideas are awful and the people up there who are published must know how you get real ideas not your fake ones. That's got a whole lot to do with the wicked imposter syndrome which rears its ugly head everywhere.
The truth is that ideas come from all over the place. Most of the writers I know don't have a hard time finding ideas; they have a hard time narrowing their ideas down to what's a good idea. I have a list I keep of all my ideas for future stories, currently there are 89 ideas stored electronically, and that doesn't include the little botecard box on my desk where I have post-it notes of all kinds of ideas. Some of these must have seemed great when I first wrote them down but now...
Well they're not a great concept or I've totally forgotten what it was that excited me about the idea.
Ideas are great and finding an idea for a novel means falling in love with a concept and then, like being in a relationship, you commit to that idea. Sure maybe you mess around with some other ideas on the side (if all parties are consenting) but you commit to seeing that idea through to the end. The biggest problem I had when I first started writing was that about halfway through a manuscript a shiny new idea would sing a siren's song and off I'd be into another project. My harddrive is a graveyard of half-finished ideas that will never see the light of day.
So, stop worrying about where to get your ideas and start focusing on your ability to follow through with those ideas you have. Don't sell yourself short, you have countless ideas bundled up inside you just ready to escape into the world, all you have to do is get them all the way done and out there.