Pokémon Sword and Shield brings over the great EXP Candy system from Pokémon: Let’s Go, Eevee! and Pikachu!.
EXP Candy is like the cousin of Rare Candy. It gives your Pokémon experience points depending on the candy’s size with XS giving the least and XL giving the most. It’s way more common to find than Rare Candy, making it perfect to grind your Pokémon quickly.
To get a bunch of EXP Candy, run Max Raids. The higher difficulty the raid, the bigger the candy reward is. So if you run a five-star Max Raid, you’ll get some L and XL Candy. Running a two-star Max Raid will yield S and M Candy. This makes doing those weak-looking four- and five-star Max Raids worth doing. Even if you don’t want to catch the Greedent, taking it down will give you some good candy.
You can also get candy by talking to the NPCs found around the Wild Area that may randomly give you some in exchange for a few Watts, but Max Raiding is the best way to reliably earn the candy.