Where do you get a master ball in lilycove department store?

You can't get a Master Ball in Lilycove Department Store in Pokémon Ruby, Sapphire, Emerald, or their remakes, Omega Ruby and Alpha Sapphire.

Master Balls are extremely rare items and can only be obtained in a few ways:

* Prize for winning the Battle Frontier: You can earn a Master Ball after completing all seven Battle Frontier facilities.

* Pokémon Contests: In some contests, you can win a Master Ball as a prize.

* Trading: You can obtain a Master Ball by trading with another player.

* Cheating: There are ways to cheat and obtain a Master Ball, but this is not considered legitimate gameplay.

You'll need to explore the other ways mentioned to find a Master Ball in these games.