Of course, that's not what you want. You want things 9-year-olds think about ninjas. You want the little-known Star of David Ninja Shuriken (which comes with instructions that only the crafty can find.)
You want cool ninja t-shirts. There's plenty. I have for you ninja pirate, only a ninja can kill another ninja, skateboarding ninja, "got ninja?", fractal ninjas (have more points), and killer coding ninja monkeys do exist, plus instructions for how to wear a t-shirt the ninja way.
Bonus for latecomers: Ninja Massage Therapist!