How do you get mystic ticket in emerald without cheating?

You can't get a Mystic Ticket in Pokémon Emerald without cheating. There's no legitimate way to obtain it in the game.

The Mystic Ticket is an item used in Pokémon Colosseum and XD: Gale of Darkness, not Pokémon Emerald. It's used to access the "Mystery Zone" in those games, which is a bonus area with unique challenges and rewards.

If you're looking to play Pokémon Colosseum or XD: Gale of Darkness, you'll need to find a separate copy of those games and the Mystic Ticket separately.