Hi Elizabeth:
Finding a "holy grail" rare Disney character can really make your day. This past October, I ran into one of these characters, Goofy's seldom-seen son Max, at
Cape May Café, and I saw Pocahontas during a visit the year prior.
You probably already know to pick up a complimentary Times Guide when you walk into the parks. 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 that you'd like to see, 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 more information regarding characters that are typically found at the four theme parks.
Happy character-hunting and enjoy your vacation.