The White Flute can be found on Route 13 (Unova) by a man in a black suit beside a house. You can then sell this to a maniac for a high price, or in Generation III, go to Route 113, walk on the grass for Volcanic Ash with your Soot Sack, and get 1000 ash, then return to the Glass Workshop and choose the flute.
Generation IV[]
The white flute helps to attract Pokémon, but it wears off in 250 steps.
Location: Route 213 (help the women looking for her suite key)
Description : A white flute made from blown glass. Its melody makes wild Pokémon more likely to appear, thus increasing the chance of finding rare Pokémon in grass.