Master Balls are incredibly rare items that can only be obtained in the following ways:
* As a reward: You get one Master Ball as a reward for winning the Johto League.
* From trading: You can potentially trade for a Master Ball with another player.
* Through glitches: In some cases, there are glitches that can allow you to duplicate Master Balls. However, these are not intended by the game and are not recommended.
While some people might try to sell you Master Balls, these are likely obtained through cheats or glitches and therefore are not legitimate.