Instructions
Earn all eight gym badges, and defeat the Elite Four and Pokemon Champion to get the National Pokedex.
Fly to Sandgem Town and head southeast, crossing through Routes 219, 220 and 221 to reach the PalPark.
Talk to Professor Oak, who gives you the PokeRadar and a new Poketch app. The PokeRadar lets you fight Pokemon from previous generations, including Ditto.
Walk or fly to Jubilife City, and exit the city to the left to reach Route 218.
Open your bag on the lower screen, and scroll over to the Key Items page.
Select the PokeRadar.
Look for rustling tall grass, which now indicates where wild Pokemon are present. Ditto has a 12% chance of appearing, but once you find one you will be more likely to fight another Ditto in your next battle. This applies to any Pokemon you battle, however.
Watch for tall grass that sparkles, indicating a shiny Pokemon. Shiny Pokemon are incredibly rare. If you have recently battled a Ditto, it is more likely the shiny Pokemon that appears will be a shiny Ditto.
Continue using the PokeRadar and searching for Ditto until you find one. This could take a significant amount of time.