I don't drink coffee, but DH does. He agrees with the other posters who say DL coffee is pretty bad. Still, if he needs something in the parks, he prefers to go with a mocha over other choices.
Pacific Wharf Cafe in DCA has mochas. I think the coffee cart near the bridge leading past Ariel's Grotto and to the pier also has them.
In DL, Blue Ribbon Bakery had them as well as the coffee cart on Main Street. I can't remember if any of the counter service places in DL offer them, but if you have a sit-down breakfast at Carnation Cafe, you can get one there.
DtD has a number of coffee options, and mochas are probably available at several of them. La Brea Bakery serves Starbucks. You probably need to go inside the quick service place rather than their kiosk to get anything other than regular coffee. The Haagan Dazs serves Peet's coffee drinks. Compass Books also has coffee drinks. The DLR hotels serve mochas and other coffee drinks in each of their coffee shops.