This is a great question. I have people ask me that all the time.
Ideas come from everywhere. I can be driving down the road in my car listening to the radio and a song comes on I really like. They think they told so much in three minutes but there is so much more I could tell about that story. :) My mind gets to turning and I end up writing a book. I think, "What happened next?"
I listen to country music so there is always some story in a song. :) I can take 3 minutes and turn it into a 200 page book :).
Ideas come from all over. I can look through stock photos, take one look at a photo, create my cover and title and then write the story. I know that is the backwards way to do it :), but as I look at the cover all these ideas start flowing. I like to start with a title, then an outline. I may not stick to the outline exactly but it gives me an idea to start with anyway.
I think as an author you just have all kind of ideas running wild in your head.