I also have my own question: Where are all the Arceus plates in ORAS?
Locations listed below:
Spoiler:Insect, Dread & Spooky Plates: Route 127
Draco, Stone & Pixie Plates: Route 128
Splash, Zap & Toxic Plates: Route 129
Fist, Meadow, Icicle & Earth Plates: Route 130
Flame Plate: Route 107
Sky Plate: Route 124
Mind Plate: Route 126
Iron Plate: ????
They are obtained underwater and require trial and error to be found since the Dowsing Machine cannot be used.
The Iron Plate is held by the Beldum you get in Steven's house after finishing the Delta Episode.