Pluto can always be found at the Disney Parks.
In the Magic Kingdom Pluto can usually be found in Town Square. In Hollywood Studios he sometime appears around the Big Hat. In EPCOT he can be spotted at the Character Spot, and at Animal Kingdom over at Dinoland USA.
In Disneyland, Pluto can be found on Main Street and Toontown. In DCA he appears on Buena Vista Street.
In Disneyland Paris Pluto is currently featuring aboard the 20th Anniversary Celebration train. He also appears frequently up by "it's a small world" and sometimes in Frontierland and Adventureland. He can also be found in Toon Studio in the Studios.
In Tokyo, Pluto meets in Toontown, and Med Harbor at DisneySea.
In HKDL, Pluto is often on Main Street USA.
On the Cruise Line, Pluto appears at select times throughout the sailings. Check your Personal Navigator for details.