What writer hasn't been asked that?
You can answer flippantly ("Anonymous e-mails!"). You can stumble around and blather for a while, sounding remarkably confused about the matter. You can say, "From everywhere, from the news, from reading." None of those answers will satisfy the non-writer who asked the question.
And, yes, the questioner is not a writer, although he may think he is. If he were, he'd understand where you get your ideas.
More to the point, he'd understand that that question betrays a fundamental misunderstanding of what motivates a writer.
A writer isn't someone who wants to have written or someone who is fascinated by the imagined glamor of the writer's life and therefore decides to become one. "Some day, when I have the time, I'm going to write a book." I bet you've heard that one, too. Odds are, people who say that will never write that book. Somehow, they'll never have the time. Or they'll have the time, but when they try writing a book, they'll realize that writing consists of lots of sitting down and, well, writing. It ain't glamor. It's words. Lots of them.
Writers don't decide to become writers and then start searching for ideas. Writers are people who keep getting ideas and feel driven to express them in writing, and so they become writers. The ideas drive the writing, and not the other way around.
Well, that's the way it is for me, anyway. I spent my childhood in a fantasy world - much more interesting than the material world outside my head - and very soon wanted to start writing my daydreams down. Oh, and of course I read oodles, so I knew that you can write your daydreams down and have other people read them. Not that I knew about the nature of the publishing biz, back then. It's surely a good thing that I didn't.
I wrote a longish essay on my Web site about why we write. It's from a different perspective than the above.