Underground or in Amity Square (i think its in one of those old buildings at amity square)
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it's ZAP PLATE and you find it underground
go to the bottom of the waterfall at the Pokemon league.
Yes, in the old chateau, but it is called the "insect plate".
Finding the Insect Plate in Pokemon Diamond requires the use of the HM Cut. It is in Eterna Forest hidden in the wall to the right of the Old Chateau.
you can get an electric next to the wind works