> After getting those 3 Grams (Letters) for Wingull on Route 13, he will fly away leaving U-Turn in his place.
> Challenger's Cave B2F, east of Protein
> Route 6: North of the southernmost bridge (requires Surf)
> Route 13: On a cliff in the southwestern corner of the route (requires Strength)
> Icirrus City: From an old lady in the Pokémon center's lobby