It's in one of the houses in Ecruteak city, you'll talk to someone and he'll ask whether or not finding treasure is the best part of adventuring. Answer yes.
Somewhere in erecutek city there is a house with a guy inside near a map he will ask if you like treasure hunting say yes and you get the itemfinder.
go to ECRUTEAK city and go to the house in between the gym and poke-center and talk to the guy and there you have itemfinder!
when you have got the itemfinder. Click start then click on bag and then press the right button on your +control pad. click on the itemfinder then click use. The itemfinder will detect any items around you that are berried.
Press use to use the itemfinder and if it dings you have found something. Walk in the direction you are facing then use the itemfinder again. continue until the itemfinder starts shaking wildly and press A facing the spot and you will find something.
somewhere
If you have the itemfinder app for the pokech, go by the bottommost building and use the itemfinder.
Just find the itemfinder press A and select 'use' then you're using it!
The itemfinder will point you in the direction of where hidden items are press A in that direction to find the item.
Simply press the A button in the direction the itemfinder is pointing to.
Pokemon emerald
There are plenty of hidden items you cannot see in the world of hoenn use the itemfinder to seek them out!
I'm sorry but itemfinder only finds items not Pokemon wish it would