I had a quick search - it seems any of the photo centers in any parks are the place to go to get the photopass+ card swapped for the voucher - they'll be marked on the park maps and I can't imagine they'd be too far into the parks. I know you are going to WDW but at DL it was at the end of Main St. I've read that it says where you can swap the voucher on the back of the dvd/voucher?
EPCOT - Camera Center
MK - Exposition Hall
DAK - Garden Gate Gifts
DHS - Cover Story
DTD - Team Mickey
And this was from the PhotoPass+ facebook page
Q. Aside from the in park PhotoPass Sales Centers, is there anywhere else I may purchase Disney PhotoPass+ at the Walt Disney World Resort and/or
Disneyland Resort?
R. Disney PhotoPass+ is available at the PhotoPass Sales Centers located inside the Walt Disney World and Disneyland Resort theme parks as well as:
At the Walt Disney World Resort the following merchandise locations have Disney PhotoPass+ available for sale*:
Team Mickey at Downtown Disney
M. Mouse Mercantile at Disney's Grand Floridian Resort & Spa
Trader Jack's at Disney's Polynesian Resort
Zawadi at Disney's Animal Kingdom Lodge
Beach Club Marketplace at Disney's Yacht & Beach Club Resort
(assuming as you can purchase it at these locations it's also the place to pick them up from the vouchers)