Walkthrough
Just west of the main area of Three Island is Bond Bridge, a path full of Wild Pokemon and Pokemon Trainers. Bond Bridge will lead you directly to Berry Forest, the same where Lostelle is being held by a certain Wild Pokemon.
Wild Pokemon
Grasses:
Pidgey (30%): Levels 29-32
Oddish(FireRed)/Bellsprout(LeafGreen) (20%): Level 31
Pidgeotto (15%): Levels 34-40
Gloom(FireRed)/Weepinbell(LeafGreen) (10%): Level 36
Meowth (10%): Level 31
Venonat (5%): Level 34
Persian (5%): Levels 37-40
Psyduck(FireRed)/Slowpoke(LeafGreen) (5%): Level 31
Surfing:
Tentacool (95%): Levels 5-40
Tentacruel (5%): Levels 35-40
Magikarp (100%): Level 5
Horsea(FireRed)/Krabby(LeafGreen): Level 5-15
Magikarp (20%): Levels 5-15
Horsea(FireRed)/Krabby(LeafGreen) (80%): Levels 15-25
Gyarados (15%): Levels 15-25
Seadra(FireRed)/Kingler(LeafGreen) (4%): Levels 25-35
Psyduck(FireRed)/Slowpoke(LeafGreen) (1%): Levels 25-35
Pokemon Trainers
Twins Joy & Meg: Level 37 Clefairy, Level 37 Clefairy
Aroma Lady Violet: Level 36 Bulbasaur, Level 36 Ivysaur, Level 36 Ivysaur
Tuber Alexis: Level 34 Staryu, Level 34 Staryu, Level 34 Krabby, Level 34 Krabby
Swimmer Tisha: Level 38 Kingler
Tuber Amira: Level 34 Poliwag, Level 35 Poliwhirl, Level 34 Poliwag
Aroma Lady Nikki: Level 37 Bellsprout, Level 37 Weepinbell
Pokemon Trainers - VS Seeker Rematch
Twins Joy & Meg: Level 49 Clefairy, Level 49 Clefairy
Tuber Amira: Level 47 Poliwhirl, Level 47 Poliwhirl, Level 47 Poliwhirl
Items
Max Repel: ItemFinder