The Black Flute is an item introduced in the Generation III games that has a better chance of the player not encountering Wild Pokémon. This item can be found in Pokémon Platinum, but not Pokémon Diamond and Pearl. This item can be sold for 200.
Ruby, Sapphire & Emerald
In the Generation III games, this item can be obtained from the glassblower on Route 113. In Pokémon Platinum, this item can be found in Mt. Coronet.
HeartGold & SoulSilver
In Pokémon HeartGold and SoulSilver, this item can be found in the Dark Cave. In the Unova region games, this item can be found on Route 13.
Omega Ruby & Alpha Sapphire