In the gate on Route 11, east of Vermillion City, you will find one of Professor Oak's aides. He will offer you the Itemfinder when you talk to him, but only if you have at least 30 Pokémon on your Pokédex.
Chat with our AI personalities
Just find the itemfinder press A and select 'use' then you're using it!
In Generation I (Red, Blue and Yellow), the itemfinder is obtained from Oak's aide in building at Route 11. You must have caught at least 30 different Pokemon before he will give it to you.
Pokemon emerald
Use the itemfinder where Mr. Fuji was in the Pokemon Tower.
Use the itemfinder.