You may purchase at Guest Services in any park, or DTD.
I had always re-upped by mail, when in-person was not an option, and was suspect of whether it would be easy on property...in July, renewed at Epcot Guest Relations. It went very smoothly. CM had obviously done it many times. If you have your AP, $60 payment (I used credit card), and (if renewing/re-upping) your DDE card/card #, they can just pull up all your info...or if new, or cannot find your card, you may print out the application from online (passholders area of WDW site) and bring it to Guest Relations
Have fun!