I have a journal of prompts that just popped into my head. (so far there are 154 starting points in it)

Anything can spark an idea. Reading the news, hearing a conversation, just the way someone looks.....

I have whole stories pop into my head sometimes just by picking a random person I don't know and trying to give them a profile.
Buses and waiting rooms are wonderful places to do this. (Just don't say your musings out loud, haha)