Hi Stacey!
The best and most current information will be found on the Times Guide available upon visiting the park. The park maps will also have a symbol for locations where typical characters meet and greets are held, as they are found all throughout the parks. You'll find a brief overview of information on character meet and greets
here for Magic Kingdom, and
here for Epcot.
Additionally, in Animal Kingdom, I've found meet and greets typically around in DinoLand, U.S.A., upon entering Africa, in Camp Minnie-Mickey, in Rafiki's Planet Watch, and around Discovery Island.
At Hollywood Studios, you'll likely find Lightning McQueen and Mater in the Streets of America, and other character favorites in places such as Pixar Place (across Toy Story Midway Mania!), Animation Courtyard,and around by the Mickey's sorcerer's hat.
Have a magical vacation!