Tickets can be purchased ahead of time, as long as available. Paying the fees though makes the $10 seats about $17. If you have time (i.e. plan ahead) you can get them by mail, I imagine.
I live close to ARCO, so last week I picked up a $10 seat at ARCO the afternoon of the game. More popular games sell out ahead of times (the $10 seats that is).
I love the $10 seats. It is a lot of fun up there. I can see the entire court. I can see fast breaks, etc developing. And the crowds are crazy at the top.