K.B.
Your best bets are AAA or Costco. But don't take the chance of purchasing anything on E-Bay or Craig's List - people sometimes try to sell "partially used" tickets on there but Disney does not allow it. When a ticket is used, they scan the users fingerprint and it gets matched to the ticket. A lot of people have tried getting "cheap" passes this way and end up getting burned.
You can also try websites like Mousesavers.com