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.
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Pokemon emerald
Just find the itemfinder press A and select 'use' then you're using it!
Use the itemfinder where Mr. Fuji was in the Pokemon Tower.
Use the itemfinder.
You should be able to receive an Itemfinder off one of Professor Oak's aides on Route 11. However, you need to have captured 30 Pokemon or more to receive it.