I can understand why people ask it but it is a difficult one to answer. I get my ideas from a combination of sources and if you even try to explain that, all you will see is your questioner's eyes glaze over as they were expecting a quick, pat answer!
Over time, I have learned to use what is around me to inspire ideas. For example, in the UK right now, we are well into autumn and I've talked about the changing seasons in my Association of Christian Writers' blog post this month on their More Than Writers page.
In From Light to Dark and Back Again, my flash fiction pieces have been inspired by moments in films, my take on well known books (for example Frankenstein and Jekyll and Hyde) and also my take on fairytales, my first love when it comes to reading and writing. My characters are nearly always amalgams of character traits, interesting things I've overheard that I've wanted to adapt and put into the mouth of a fictional person, and physical qualities (some good, others less so!).
So there is no one "go-to" place for ideas, nor should there be, and I guess the only answer to the question is to tell your questioner you get your ideas from many different places. If they are really keen to know more, you can expand the conversation. If not, well there's your quick answer and you haven't bored them! Result!