It is found in Ecruteak City Gym
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You can find TM 30 shadow ball in the last floor of MT.PYRE and in the Pokemon tower
go to celadon city and buy coins then go to where you get the prizes, shadow ball is one of them
Give Kurt one of the Apricorns. Don't recall which one though...
Route 210, right before Celestic Town. You need to ride your bike across a few planks.
You find a wild Pikachu. You can either catch it, or use thief on it (NOTE: Pikachu has a 5% chance of having a light ball, and a 50% chance of an Oran berry). Use it well!