My answer is always the same. The best source for writing a story is never far away . . . you are it!
Begin to pay attention to your life, especially your emotions. Discover what makes you happy or sad, what makes you angry, what makes you laugh or cry. You will find an idea in your daily experiences.
Be sure to make a quick note of your discovery or draw a simple doodle. If you do this, the next time you sit down to write you will find many gems of inspiration.