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From Lance after you infiltrate Team Rocket's Secret Base in Mahogany Town.
you have to defeat team rocket at their secret base at the game corner and the boss drops it after you defeat him
go to mahogony town and find their secret base in the house with the old lady and beat them there with the champion from the Pokemon league lance
Get the TM secret power teach it to a Pokemon then find either a tree with vines, an indent in the wall or a big bush use secret power on these to create your own secret base.
in the underground.