There are two ways to get Focus Band in Ultra Sun/Ultra Moon.
1(Finite Method): Talk to the Police Officer behind the desk in the Route 9 Police Station after defeating all of the Ditto Five. (Source)
2(Infinite Method): You can get Focus Band from wild Machop and Machoke, both having a 5% chance to hold them. (You can increase the odds to 20% if you have a lead Pokemon with Compound Eyes) (Source) (Compound Eyes)
There's also the option to have someone trade you a Pokemon holding a Focus Band if you know someone locally who plays Sun/Moon/Ultra Sun/Ultra Moon.
Hope this helps!
