In any of the gatehouses (I don't remember where it is in GSC), talk to the security guards. If there is something you can get in there (like an Exp. Share), he'll say something along the lines of:
"Hey, one of Prof. Oak's assistants is upstairs looking for you." Something of that nature. Then, go up the stairs (they'll usually be opposite the guard, like from bottom to top and vice versa), and find the guy wearing glasses. Talk to him, and if you have enough Pokemon in your Pokedex, he'll give you whatever item you needed.
If you remember like HM05 and Itemfinder and Exp. Share from RBY, it's the same procedure. I'm just not sure what the requirements are as far as Pokedex, or where it is.
But I bet that this site has a guide to find it. Go through the walkthrough if you're so curious. I know that they usually put notes of that magnitude in there.
Hope this helps!
It's great to complete a Pokedex! So I'll need some help...
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