Despite Stitch's immense popularity, he seems to be less frequent these days.
In WDW you can try to get a picture with him at the Dance Party that happens throughout the day in Tomorrowland, but that can be difficult. You can usually find him wandering around the main entrance to EPCOT as well.
In Tokyo Stitch usually meets in Arabian Coast, which we find quite odd.
At Disneyland Paris he can sometimes be found in the afternoons in his Elvis outfit near his Stitch Live! attraction.
In California, Stitch seems to be quite rare these days, though if you pay for the meal, you can usually (but not always) get him at the Surf's Up breakfast at the Paradise Per Hotel.
In Hong Kong, Stitch used to meet in Tomorrowland, but seems to have vanished as far as we can tell.
Select Disney Cruise Line sailings sometimes feature Stitch. You should check your Personal Navigator for details.