From Bulbapedia, the community-driven Pokémon encyclopedia.
The Itemfinder is an item used in the Pokémon games to find hidden items. In those games based in Kanto, it is given out by one of Professor Oak's aides in the lookout on Route 11 if the player has 30 or more Pokémon caught in their Pokédex. In Johto-based games, it is given out by a man in Ecruteak City. In all games it is a Key Item, meaning that after Generation II it can be set to the Select button and in all generations cannot be sold.
In Sinnoh, the Pokétch's Dowsing Machine app replaces the functions of the itemfinder.
In the TCG
The Itemfinder in the TCG series is a trainer card that allows the user to retrieve a trainer card from their own discard pile and put it into their hand. Two other cards (besides the Itemfinder itself) must be discarded to do so, however.