Ideas can come from anywhere.  As a stock photographer, your job is to make them come from everywhere.  There are 2 steps to coming up with great stock ideas:

1) Create or visualize the concept.

2) Record the idea in a medium other than your brain.

Create or visualize the concept

The area most stock photographers struggle with is the creation of new stock ideas.  If you are sitting at your desk everyday, you won’t come up with many ideas.  Ideas for stock are based on activities and the world.  Activities are based in magazines and on websites as well but mainly, ideas are what life is - go where there’s life and your ideas will flow.  Visit a beach.  Imagine your model walking on the beach, swimming, maybe there’s a volleyball game going on.  Anywhere you see people doing anything - that’s an idea.  Maybe a family is having a bbq and another a picnic.  Two ideas?  What about twenty?  Photos of the bbq grill, photos of the meat cooking, photos of veggies cooking, photos of people eating, photos of a family around the food, photos of the picnic table setup, photos of the charcoal while it’s hot & red.

Record the idea in a medium other than your brain

If you don’t carry a notebook, start.  Write down EVERY idea you have - you may not have it again, at least for a long time.  You don’t want to let your ideas slip - either abstract & conceptual or specific like “woman wiping sweat off her forehead, woman drinking from a clear bottle” whatever.  Just write it down, type it out, record it somehow that doesn’t rely on your brain.