Unfortunately, you can't! The last event for Zeraora in USUM was from 3 May 2019 to 26 May 2019. The last one in America was 19 October 2018 - 9 November 2018. Hence, there aren't any ways to get it in USUM right now without trading!
Right now, it's even impossible to obtain it in Pokémon Home. There's no way other than trading with a friend to get Zeraora!