They usually sell tickets by the ticket collections booths (next to the match day magazine sellers and before the charity buckets), and just inside the entrances to the South Stand.
I'm surprised you haven't heard their cries of 'Withdean Wager, only a pound!'
I don't mind doing the WW, because if they sell 1000 tickets, you have a 1/1000 chance of winning £500, whch is better than the Lotto offering £10 at 57/1