Hi Amanda:
Locations, times, and characters change often. Disney World has some general information online regarding character greetings at
Magic Kingdom,
Epcot (including the popular
Epcot Character Spot),
Hollywood Studios, and
Animal Kingdom (including
Camp Minnie-Mickey Greeting trails)
When you get to the parks, pick up a complimentary Times Guide. Along with providing information regarding show times, parades, and special events, the Times Guides generally include character greeting times and locations.
If you have a particular character in mind, your best bet is to stop by guest relations at any of the theme parks and ask them to call the "character hotline" for you. The character hotline will be able to tell you if, where, and when any particular character will be in the parks that day.
Another resource is to visit website allears.net (not an official Disney site) and use the search engine to search for "characters at a glance." You can find information regarding characters that are typically found at the four theme parks.
Enjoy your vacation.