Pretty much anywhere that sells APs/PAPs can take care of it for you.
If it were me, I'd go to the MarketPlace Guest Relations Office in Downtown Disney. Those CMs are reported to be very well trained and well versed in all of the ticket transactions. They've also been reported to be very, very nice about helping guests with DDE purchases. You're also less likely to run into lines there or the "impatience" of other guests in line behind you who are waiting to get right into a park.
Best wishes -