Celery is in Mr Herriman's office. This question has been asked a billion times, and a quick search would have yielded the answer. That's not a snark at you as much as a snark at the game designers, because if they hadn't made it so hard to find the stupid celery people wouldn't have had to ask a billion times.
Just remember to use the search function, lots of people have been playing the game and posting here so you'll find lots of info that way.