In Pokemon Ultra Sun, I accidentally knocked Necrozma out. Where would I get it again?
Assuming you mean when you encounter it at Mount Lanakila right before the Pokémon League (where you actually have a chance to catch it), YES. Just become champion/defeat the League again and it will be waiting in exactly the same location.
Yes you have to become champion and defeat the league again and Necrozma will be in the same crater where you found it. I highly reccomend to save the game before battling Necrozma, so if you knock it out again, you can just try again from where you saved.