You will not find a listing where they're looking for production assistants (PAs), at least not in the same manner that you would find a "normal" job. The easiest way to become a production assistant is to start on a student film or indie film. You find these opportunities by doing internet searches for your state like this "new york student film crew call." Once you find a crew call, you submit to it with a simple email to whatever email address is available to you. Most likely, this gig will be unpaid, but that's fine. You're initially not doing it to get paid but to get experience. Once you become a production assistant on a major set, that's when the money comes in.