I am training a nidoran(boy)
Non-repeatable - Route 2 (South of the eastern gate; requires Cut) - Mt. Moon (1F; northwest area of floor) - Mt. Moon (B2F; hidden item southeast of the Super Nerd) - Rocket Hideout (B2F; northwest of tile maze) - Pokemon Mansion (1F; hidden item on fifth pillar northeast of entrance)
Moon Stone (Bulbapedia)
You can find one at Mt. Moon, the Rocket Hideout and on Route 2.