generally, as in many things in music, orchestrators work freelance. Music is a low paying field, so the pay isn't great, but its not terrible

If you look carefully in the endless credits scroll you can see the credits for the orchestrator(s), if the composer doesn't do the orchestration themselves. Herbert Spencer, for instance, is credited for orchestration of Star Wars Ep. 4, A New Hope. He worked as orchestrator on a lot of other John Williams movies and was a film composer in his own right as well.