Alolan Forms were introduced back in Pokemon Sun & Moon, giving New forms to a certain amount of Kanto Pokemon. Alolan Forms are very different than a Pokemon's Kantonian Form, since said Pokemon evolved differently in the Alolan Region Climate. Almost all Alolan Forms have differences in appearance compared to the same Pokemon you will find in the Kanto Region, and this change usually seems these Pokemon taking on new types. Some of these Pokemon will even have different Stats compared to their Kanto Forms.
Since Alolan Forms can only be found in the Alolan Region, you can't find a Pokemon in their Alolan Form in the wild. Instead, you will only see given Pokemon in their usual, Kantonian Forms. However, throughout the Kanto Region, you will find trainers that are willing to trade you their Alolan Form Pokemon. In exchange, they will each be looking for the same Pokemon they are trading you, except in their Kantonian Forms. Outside of transferring Pokemon in their Alolan Forms from Pokemon GO or obtaining them by internet trades, this is the only other known way to obtaining the Alolan Form Pokemon within this game.
Alolan Form Pokemon Trade Locations
Rattata: Cerulean City Pokemon Center
Geodude: Vermilion City Pokemon Center
Diglett: Lavender Town Pokemon Center
Sandshrew: Celadon City Pokemon Center
Raichu: Saffron City Pokemon Center
Marowak: Fuchsia City Pokemon Center
Grimer: Cinnabar Island Pokemon Center
Exeggutor: Indigo Plateau Pokemon Center