At the Power Plant.
It's in mistys gym
head East of Cerulean City then surf a little.
Zapdos is located outside of the Power Plant in Kanto.
he comes back to the power plant every day
Power Plant
you have to have all 16 badges after that zapdos will be right outside the power plant
you meet a team rocket member who hides the rotor in a ring
When you return the machine part to the power plant.
It is on the left side of Saffron city you can go there after solving the power plant problem.
you have to look in places where a legendary might live (example: Zapdos=power plant)
you have to have completed the power plant mission and he will be outside the power plant on the left