Flash can only be learned by:
* TM31: You can find TM31 in various locations throughout the game, including:
* Goldenrod City: Inside the Game Corner.
* Victory Road: In a hidden room behind some boulders.
* Route 47: Given to you by an old man.
* Breeding: Some Pokemon learn Flash through breeding.
Note: While many Pokemon can learn Flash, it's a move that is primarily useful for its ability to illuminate dark caves. It's not a particularly strong attack move.