They'll release the schedule around mid-August. You can buy the shirt while you're there. Also, around the time that they release the schedule, they'll also announce late night shows around Austin. Attend one with your favorite artist. It's much more intimate than ACL since you'll be in a small venue. These days, I skip ACL and only attend late night shows.
They haven't dropped the exact schedule for individual artists yet, but will likely do so in the weeks leading up to the festival if the previous years are anything to go by. Probably the same case with this years merch as well.