As soon as you have surf capabilities
Pokemon Sword and Shield has a ton of layers underneath the surface, and a lot of them are predicated on the Wild Area.
As soon as you gain the ability to surf (along Route 9 below Circhester, as part of the story), you’ll want to head to the Lake of Outrage in the northern Wild Area.
To get there, simply fly over to the northern Wild Area, right below Hammerlocke in the image above. Facing the vista (with the stairs to Hammerlocke behind you), get on your bike, veer right and pedal until you see a lake.
Cross that lake (which will often have great creature spawns), and go up the ramp until you see a series of stones. Jackpot! Each of those rock structures corresponds to a stone (Fire, Leaf and so on), which are on the ground in plain shiny view right behind them.
There’s another bonus, too. Depending on the weather, a rare Pokemon might appear there and save you from using a stone or save time on a lengthy evolutionary run. For instance if it’s raining you might find Vaporeon, or if it’s night you mind find Umbreon. Rotom or Ditto can spawn here too. Good luck!