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!
yay!! the vivian girls were adorable. unfortunately, they're sizeists... there was no XL t-shirt available at the merch table, and i was informed in a way that inferred there never had been! :(