[S/M] HOW TO GET MORE BEAST BALLS

[Eugenio]

While you're hunting down the Ultra Beasts give every single Beast Ball in your bag to your Pokemon to hold. Talk to Looker and ask him about the Beast Balls, when he sees you have no more in your possession he will give you another one. Repeat the process to get as many Beast Balls as your heart desires.

I found this out thanks to this Youtube video.