Best tip if you're looking for a specific character is to ask a CM so they can call CHIP, which is a character hotline.
In Epcot you can find characters all around the world: Mexico - Donald with his friends from the Three Caballeros movie, Norway - has a princess dining, China - Mulan, Germany - Snow White with one of the Dwarfs, France - Aurora and Belle with the Beast, Marocoo - Aladdin characters, England - Mary Popins and Alice, sometimes characters from Winnie the Pooh inside the English shop.
A few years ago I met Chip and Dale at the International Gateway as well, and there was hardly any guests there so we got to spend plenty of time with them, I'm not sure if they do that anymore though. And each park also have characther spots marked in the park maps where you'll be able to meet characters all the time.