After you get the Gym Badge from the Stow-on-Side Gym Leader, you can reach your next destination by going through the Glimwood Tangle.
Explore the Glimwood Tangle
The pokémon that can be found in rustling patches of grass in this area are Hattrem, Indeedee, Mogrem, Oranguru (Shield only), Passimian (Sword only), Phantump, Galarian Ponyta, Shiinotic, Sinistea, Spritzee (Shield only), and Swirlix (Sword only).
If you check on the reddish-pink mushroom, you will have a wild pokémon encounter with Impidimp. There are other mushrooms in the area that you can check on to find Impidimp.
If you check on other mushrooms here, you can use them to temporarily light up dark areas.
Go north to where the path splits. If you go west, you can optionally battle Cook Derek, who has a level 33 Milcery, a level 33 Sinistea, and a level 34 Shiinotic. You get 4080 Pokédollars for winning.
From there, go south into the dark area and follow the path, and you will find a bag of Bright Powder.
Go east to jump down from the short ledge. Then go south, and when the path splits, go east. Keep going east and you will find a sparkle. Check on it to get 2 Full Heals.
Then go south through the nearby grass and follow the path. It will lead you to a Big Root.
Go east, then north. When the path splits, keep going west. When it splits again, keep going west and you will find TM24 Snore.
Go east, and if you want to skip a trainer battle, you can go up the first of the northern paths that you encounter. Otherwise, you can go all the way east, then north, to battle Daring Couple Robert and Jacqueline, who have a level 34 Kirlia and a level 34 Ninetales. You get 7344 Pokédollars for winning.
Go north, then go west, and stay south of the tree root that splits the path. You will find 2 Hyper Potions.
From there, you can go south and optionally battle Madame Judy, who has two level 33 Indeedee. You get 6600 Pokédollars for winning.
Go west through the grass and a Mogrem will attack.
Go west from there, then north. At the north end of the path, there is a sparkle. Check on it to get a Luminous Moss.
Go west from there and follow the path to find TM56 U-Turn.
From there, go east and jump down the two ledges. Then go northeast and follow the path to reach Ballonlea.