Brainstorming.
I write Yu-Gi-Oh fiction. A lot of ideas come out of me when I go around the net, looking at websites where there's information on the cards and the anime. I look at these hundreds of cards, and eventually, an idea pops up.
I have another way of getting ideas: I keep a logbook. Every time I think of something interesting, I write it down. I've gotten more than one good idea like this. Big ideas often come out of small ideas. Did you know that the Harry Potter books came out of notes that J.K. Rowling wrote down on napkins while she was drinking (and often drunk) in British coffee houses? Never overlook a small idea, because it can turn into something big.