Kinda hard to answer. That’s because we get our ideas from everything and everywhere. A commercial might spark a stray thought. A movie or television program often produces ideas. Reading books … whether fiction or non-fiction helps.
Then there’s that deep, dark part of our subconsciousness that speaks to us. Yes, we hear voices in our heads. Characters whose stories are just waiting to be written, to have their moment in the spotlight. They often chatter away to us until we’re forced to write their story. Doesn’t matter that we’re in the middle of another project. Still gotta stop and at least make notes or even write entire scenes so that the ideas aren’t lost.
Personally, I think that’s half the fun of writing. Always having that new idea waiting in the wings … ready to be written.