Master Balls are exclusive to Pokémon Gold and Silver.
In those games, you can find a Master Ball in the following locations:
* Goldenrod City: In the Game Corner, you can win a Master Ball by obtaining 49 tokens and exchanging them for a prize. This is the only way to get a Master Ball legally in Gold and Silver.
* Trade: You can also get a Master Ball by trading with someone who has one.
Since Crystal is a separate game with a different storyline and item availability, the Master Ball isn't included.