It's possible you're thinking of a different item, like:
* Lucky Egg: This item increases the experience points earned by a Pokemon after a battle. You can get this from a Pokemon Trainer in the Veilstone Department Store.
* Incense: This is a general item type that affects the wild Pokemon you encounter, not the player's luck directly. You can find different Incenses throughout the game, like the "Spicy Incense" which increases the chance of encountering a Fire-type Pokemon.
If you're looking for a specific item, make sure you double-check the name and its effect.