It is hidden in the area that the guy is at. When you find it, the guy says its not his glasses so you can keep them. They actually are the BlackGlasses hold item for pokemon. You must use the item locator to find it, or just walk around and select random pieces of the ground until you find it. Hope this helps!
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If a guy outside of the Rusturf Tunnel asks you if you can find his glasses. You can use the Itemfinder to find some Blackglasses. They won't be his, but the glasses are very useful and they increase the strength of Dark-Pokemon moves. Hope this helps!
Use itemfinder at the spot where he talks to you. find the black glasses and talk to him. you will never see him again.
on the floor near the guy on the grass or on the cliff in the area of where youy find the person
well there are the black glasses which are hidden in the area where the guy said he lost his glasses. but when you talk to him he'll say they aren't his glasses. but there are no other lost glasses on route 116. so basically the guy will pretty much never find his glasses. but at least you can keep the black glasses which is good if ur Pokemon have any dark type moves.
the real glasses: you cant find them. if you show the blackglasses he depart from there. you find blackglasses with the itemfinder