Minnie Mouse can be found regularly in set locations in every Disney Theme Park.
At the Magic Kingdom you can find her at the Silly Sideshow. In EPCOT she is always at the Character Spot. In Animal Kingdom you can meet her at the Adventurer's Outpost on Discovery Island. In Hollywood Studios Minnie can often be found inside the Animation building.
In Disneyland Minnie can be met frequently at her house in Toontown or on Main Street.
In Disneyland Paris she can be regularly seen randomly around the park (usually Main Street or Fantasyland). At the Studios she usually appears in Production Courtyard.
At Tokyo Disneyland, Minnie can be found in lots of different place that seem to change regularly! You can nearly always find her in Toontown and American Waterfront, though.
At Hong Kong Disneyland, Minnie meets her fans regularly on Main Street, in Fantasy Gardens or in her Western gear at Grizzly Gulch.
Aboard the Cruise Line, Minnie appears regularly throughout the sailings. Check your Personal Navigator for details.