Hi Rebekah:
Characters are always subject to change and new characters are introduced every so often. Plus there are rare characters, like Goofy's seldom-seen son Max, that appear every so often. Disney World provides some general information on their website about character opportunities in the
Magic Kingdom,
Epcot (including
Epcot Character Spot),
Hollywood Studios, and
Animal Kingdom.
Disney's restaurant list can be sorted by character dining, which will give you more insight on characters that appear in restaurants.
Likely your better bet would be 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 and the restaurants.
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.
Enjoy your vacation.