Arceus Plates
Arceus' ability is Multitype, i.e. when you make Arceus hold a plate, Arceus' type changes to the plate's. There are sixteen plates in all (one for each type, minus the normal-type). Without a plate, Arceus is normal-type.
Meadow Plate
Found buried underneath a waterfall on Route 210
Flame Plate
Found on Stark Mountain
Splash Plate
Found buried on Sandgem Beach (Route 219)
Insect Plate
Found buried to the right of the Old Chateau (Eterna Forest)
Zap Plate
Found buried to the left of the Vista Lighthouse in Sunyshore City
Sky Plate
Found buried at the bottom of the Pokemon League waterfall
Icicle Plate
Given to you in the house on the left on Route 217, after bringing HM08 Rock Climb to the owner
Stone Plate
Found in Mt. Coronet, above the Waterfall
Earth Plate
Found in Oreburgh Gate, Level B1F
Toxic Plate
Found buried in the Great Marsh, Area 2
Fist Plate
Found on Route 215
Mind Plate
Found in Solaceon Ruins
Dread Plate
Found in Lost Tower
Spooky Plate
Found in one of the houses in Amity Square
Draco Plate
Found buried behind the Dialga/Palkia statue in Eterna City
Iron Plate
Found in Iron Mountain