Readers are very fond of asking this question. And whenever I answered, "Anywhere", they aren't satisfied.
I'm giving some examples on how I get my ideas. By playing what ifs, daydreaming or even, dreaming. By observing. By experiencing.
Playing what ifs is easy, and I'm sure most of you have your fair share of them. Just use imagination to depict a scene--the more outrageous, the better! Try to imagine becoming someone else, changing your way of thinking, putting yourself in somebody's shoes. Daydreaming can be healthy and fun, as long as you know how to differentiate reality and fantasy.
Observing can be both enlightening and invigorating. There are some ways to do this. You can read or watch TV/movies. Or you can go to the crowd and observe. Use your imagination. Make some stories about the ones you've seen. This is a fun exercise as well.
How can you experience things? Well, there are times when you need to join the fun instead of waiting patiently in the corner! Ever wondered how tough becoming a singer can be? Enroll yourself in a vocal class. Take a hike and stroll in the park, try to enjoy the very small things like the sound soft rustle of leaves and the sight of soft-petaled roses.
And most importantly, BRING YOUR NOTEBOOK! Write down your ideas. Even if they refuse to come, be positive. Life is meant to be enjoyed to the fullest, and when you're happy, ideas will come running to you.