I would call the venue. They might release some at the door, but it seems like they've been doing that less nowadays.
Or sometimes (sometimes) you can get lucky and they'll sell you a ticket once the headliner has gone on and a significant amount of people have left the show.
I saw the Vivian Girls a couple months ago in Santa Cruz and they are super awesome, the crowd energy was intense in that tiny place. They were also really sweet at the merch table afterwards!
I heard there will be two big and well-known "suprise guests" this year. Does anyone know more details? I'm crossing fingers that I will be able to go to Coachella this year.