You can dig Iron Plate up underground or find one buried somewhere on Iron Island.
* You can find it on a rock up a water fall in the mountains where you would find a dratini.
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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".
There is not such item called "black steel orb".
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.