If you do ticket at SFO, beware that the ticket purchase window keeps moving. I was there on Sunday and it actually changed mid-transaction for me (I had to follow the agent from one window to the other).

The two most common places for this to be (this week anyway) is
1) The old premier lobby on the right side of the main security checkpoint and elevators.
2) The rightmost line on the left side of the main security checkpoint.