It is located underneath the game corner in celadon city.
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saffron city
That building is in saffron city.
You need to get the Silph Scope.
The ghosts can't be fought in their current forms. To beat them you have to see them and you can when you get a Silph Scope. You get the Silph Scope in Celadon City somewhere (No spoilers unless u want them)
You get it from one of professor Oak's aids i believe, other wise, you get it from someone from the Silph Co.