The WDW Shop Disney Parks app shows Duffy clothes available at:
Animal Kingdom: Discovery Trading Company (Right side - if facing Tree of Life)
Epcot: Der Teddybar (Germany Pavilion), Image Works (Figment store), and Disney Traders (World Showcase Pavilion on path toward Mexico)
Hollywood Studios: The Studio Store (Animation Courtyard, left side, close to entrance to Star Wars: Launch Bay and the bathrooms)
Magic Kingdom: Big Top Collectibles (Fantasyland), Emporium (Main Street)
Some costumes are available at some Resort Gift shops (Contemporary, Polynesian, Pop Century, Port Orleans: Riverside)
I don't believe the app works internationally, but once you are in the states, if you have a smartphone you might want to see if you can download and install it.