Here I am necroposting on the second oldest question on this whole site.
Rattata can be found in Routes 1, 3, 4, 7, 9, 11, 22, 29, 30, 31, 32, 33, 34, 38, 39, and 46, Sprout Tower, Union Cave, Burned Tower, Bell Tower, Mt. Mortar, and Tohjo Falls.
https://bulbapedia.bulbagarden.net/wiki/Rattata_(Pok%C3%A9mon)