The park map will list specific M&G locations, but what it won't list is "random" M&Gs.
In
Disneyland you have "random" characters on Main Street throughout the day. This includes Mickey and the gang and a few surprises. The Princesses and Tinkerbell have assigned M&G locations just outside the hub. Pooh and friends meet in Critter Country outside their ride. And Mickey and Minnie can be found in/near their houses and the gang can be found wandering Toon Town. Those are the only specific M&G at DL, unless there's an event like the Spring Roundup. Characters like Peter Pan and Alice can be found around Fantasy Land, but they do not have specific locations. I've seen Brer Fox and Bear in Critter Country too. You can ask a character CM (those working with characters) for specifics about a character you're looking for.
At DCA "random" meetings of Mickey and friends in period clothes can be found on Buena Vista St, and Oswald has a M&G location there too. Disney Jr. Characters are often found outside the theater and Incredibles and Monster's Inc. characters can be found on Buena Vista. Anna and Elsa now meet at the Character Close up, they're the only ones with FP right now, though Olaf may be getting something different with his Snow Fest closing. Lightning McQueen and Mater rotate in Carsland outside the Cozy Cones and Red can be seen there as well as DJ who does a dance show. Donald is often found at the beginning of the pier in a gazebo. The Toy Story gang has a M&G set up near the Garden Grill. You can sometimes find characters from UP at the playground in Grizzly. Minnie Mouse, dressed as a pilot, can be found in the airfield after her show and sometimes at other times. And Flick can be found in Bugsland. That's all I can think of right now.
Each hotel and both parks have a Character Meal with various characters too. Again, if you're looking for someone specific, ask a CM.
