In-Game Trades

Like past Pokémon games, there are numerous characters that will do an in-game trade, offering you a Pokémon in exchange for another. However, Pokémon Let's Go, Pikachu! & Let's Go, Eevee! changes this somewhat and has it so these trades can be repeated as much as you want. This is due to the fact that these trades offer you the Alola Forms of Kanto Pokémon

#019 Rattata

Location: Cerulean City

In the Cerulean City Pokémon Center, a trainer will offer you a trade. In exchange for a Rattata, they'll give you an Alolan Rattata.

Rattata
Level 12
OT:Tatianna
ID:121106
?? Nature.
Date Received. a Link Trade. Received at Level 12
Tail Whip
Quick Attack
Focus Energy
Bite
 
Trader requests a Rattata
#074 Geodude

Location: Vermilion City

In the Vermilion City Pokémon Center, a Hiker will offer you a trade. In exchange for a Geodude, they'll give you an Alolan Geodude.

Geodude
Level 16
OT:Higeo
ID:551873
?? Nature.
Date Received. a Link Trade. Received at Level 16
Bide
Rock Throw
Stealth Rock
Take Down
 
Trader requests a Geodude
#050 Diglett

Location: Lavender Town

In the Lavender Town Pokémon Center, a trainer will offer you a trade. In exchange for a Diglett, they'll give you an Alolan Diglett.

Diglett
Level 25
OT:Digette
ID:520159
?? Nature.
Date Received. a Link Trade. Received at Level 25
Fury Swipes
Dig
Slash
Sucker Punch
 
Trader requests a Diglett
#027 Sandshrew / #037 Vulpix

Location: Celadon City

In the Celadon City Pokémon Center, a trainer will offer you a trade, but this trade varies between game version. In Let's Go, Pikachu!, she will request a Sandshrew while in Let's Go, Eevee! she will request a Vulpix. In exchange, she'll give an Alolan Sandshrew and Alolan Vulpix respectively

Sandshrew
Level 27
OT:Nicholice
ID:703019
?? Nature.
Date Received. a Link Trade. Received at Level 27
Swift
Mirror Coat
Fury Swipes
Ice Punch
 
Trader requests a Sandshrew
Vulpix
Level 27
OT:Nicholice
ID:703019
?? Nature.
Date Received. a Link Trade. Received at Level 27
Confuse Ray
Aurora Beam
Dazzling Gleam
Ice Beam
 
Trader requests a Vulpix
#026 Raichu

Location: Saffron City

In the Saffron City Pokémon Center, an Ace Trainer will offer you a trade. In exchange for a Raichu, they'll give you an Alolan Raichu.

Raichu
Level 30
OT:Psytrice
ID:940711
?? Nature.
Date Received. a Link Trade. Received at Level 30
Thunder Shock
Growl
Tail Whip
Double Team
 
Trader requests a Raichu
#105 Marowak

Location: Fuchsia City

In the FuchsiaCity Pokémon Center, a Gentleman will offer you a trade. In exchange for a Marowak, they'll give you an Alolan Marowak.

Marowak
Level 38
OT:Genmar
ID:102595
?? Nature.
Date Received. a Link Trade. Received at Level 38
Rage
Will-O-Wisp
Bonemerang
Thrash
 
Trader requests a Marowak
#088 Grimer / #052 Meowth

Location: Cinnabar Island

In the Cinnabar Island Pokémon Center, a Punk will offer you a trade, but this trade varies between game version. In Let's Go, Pikachu!, he will request a Grimer while in Let's Go, Eevee! she will request a Meowth. In exchange, she'll give an Alolan Grimer and Alolan Meowth respectively

Grimer
Level 44
OT:Darko
ID:000219
?? Nature.
Date Received. a Link Trade. Received at Level 44
Bite
Screech
Crunch
Acid Armor
 
Trader requests a Grimer
Meowth
Level 44
OT:Darko
ID:000219
?? Nature.
Date Received. a Link Trade. Received at Level 44
Screech
Slash
Nasty Plot
Play Rough
 
Trader requests a Meowth
#103 Exeggutor

Location: Indigo Plateau

In the Indigo Plateau Pokémon Center, a trainer will offer you a trade. In exchange for a Exeggutor, they'll give you an Alolan Exeggutor.

Exeggutor
Level 46
OT:Exemann
ID:060310
?? Nature.
Date Received. a Link Trade. Received at Level 46
Barrage
Hypnosis
Confusion
Stun Spore
 
Trader requests a Exeggutor