When Pokemon Sword and Shield announced it wouldn't feature some classic Pokemon, there was one we never doubted would return: Ditto. You see, the cute little pink blob is a practically vital part of Pokemon breeding - and so if you're going to partake in that side of Pokemon Sword and Shield, you're going to want to know where to catch a Ditto.
If you don't know why Ditto is special, the answer is pretty simple: it's known as the Transforming Pokemon, as it copies whatever Pokemon it meets. If it faces off with a Corviknight in battle, it too can become a Corviknight. This also works for breeding - so if you only have access to one Pokemon, as is the case for some like the Starter Pokemon or Charmander, you can breed it with a Ditto in order to get two or more. We'll let you ponder the science of that...
How to find and catch Ditto in Pokemon Sword & Shield
Anyway, Ditto has always been a rarer, less common Pokemon - and that's still the case in Sword and Shield. In fact, Ditto only spawns in one place - the Lake of Outrage, which is the zone in the top left of the Wild Area. On the world map, that's just to the west (left) of the Hammerlocke Hills, the very northern-most point of the Wild Area.
The Lake of Outrage is actually located across the water from the Giant's Cap area, and that means you'll need the Rotom Bike upgrade that lets you ride across water in order to access it. You get this during the natural course of the story - it'll be rewarded to you shortly after you first travel down Route 9, so well before you complete the gym challenge and reach the Sword and Shield Post Game.
Cross the water and climb up the curving mountain path that dominates the other side of the Lake of Outrage. Up here there are patches of grass. Lots of high level Pokemon spawn here, but so does Ditto. As an aside, the evolution stones used to evolve Eevee can be found up here, too.
Ditto can appear out on the overworld in this grass. It's a short little thing, so keep an eye out for that pinky-purple blob sliding around. It can also appear as a random encounter spawn, but the chances of that are far lower. It likes normal weather, so if it isn't a regular, sunny day, consider coming back later.
In truth, though, you shouldn't struggle to find a Ditto in this area. We managed to find four in about fifteen minutes. They all come in at over the level 50 mark - so you'll need some half-decent Pokemon in power terms and a bunch of Ultra Balls in order to catch them. Ditto will transform into the first Pokemon you deploy - so keep that in mind when considering what you'd like to fight and feather in order to catch it. A transformed Ditto will revert to its true gooey form once caught.