I take my inspiration from reading books, playing other games, just walking through school or some other casual activity. Music sometimes, too.
What's also interesting I think is how to handle this inspiration. What I found usefull is working out a theme a bit into something workable, and then if I can't use it straight away store it in some form (usually sketches, sometimes some test maps as well) for later use. I've got several themes and idea's around so when I'm done with Mudanchee I'm can simply pick the best one of them and get going again immediatly.
Don't throw away something because you think it's bad - perhaps if you come back at it later you'll see more possibilities to turn it into something good.
//CP - Inspiration handling, 1:1