You wait until you beat the elite four and professor Elm will give it to you when you beat the elite four.
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You get it from a Fishing Guru on Route 12 at the Kanto region.
talk to lots of fishermen and one will give u one
every time you see it say got a nibble press a and soon you will hook a Pokemon
you go to the docks near jubilife city go back into the house talk to the fishermen he will give you a fishing rod
no you can trade soulsilver pokemon to black