Alright, a bunch of people think that many of the Arceus plates are only obtainable underground, but so far, thanks to various guides on the internet and my own intuition, I've found every plate in the overworld.
Here are thier locations, use the itemfinder to make things much easier:
Flame Plate: Within Stark Mountain, in the left side of the big cave part.
Splash Plate: Route 220, take the left-south route, it's on the beach south of 2 bikini girls.
Zap Plate: Sunyshore City, in front of the lighthouse.
Meadow Plate: Route 210, down the stream and up the first waterfall, hidden in the wall along side an empty patch of land.
Icicle Plate: Route 217, talk to the man inside the north-east house.
Fist Plate: Route 215, near the beginning of the route, past the cut trees.
Toxic Plate: In the Safari Zone, take the train to Area 1, then go the the most northeastern corner of the Safari Zone and search there.
Earth Plate: Oreburgh Gate, go to the basement and surf all the way to the left. It's somewhere there by a strength boulder.
Sky Plate: Pokemon League, under one of the statues just outside the exit of victory road.
Mind Plate: Solaceon Ruins, at the end among the stash of items.
Insect Plate: Eterna Forest, in the right wall just outside Eterna Old Chateau.
Spooky Plate: It's in Amity Square. That place where you walk with your cute pokemon. In one of the buildings.
Stone Plate: Mt. Coronet, when you reach the snowy area, look for an entrance that leads you into a cave with a waterfall. Climb the fall and it should be up there somewhere in a tiny cave.
Draco Plate: Behind the Dalgia/Palkia statue in Eterna City.
Dread Plate: Eterna Old Chateau, inside in the top left room.
Iron Plate: Iron Island, somewhere in the lower part of the cave.
Here are thier locations, use the itemfinder to make things much easier:
Flame Plate: Within Stark Mountain, in the left side of the big cave part.
Splash Plate: Route 220, take the left-south route, it's on the beach south of 2 bikini girls.
Zap Plate: Sunyshore City, in front of the lighthouse.
Meadow Plate: Route 210, down the stream and up the first waterfall, hidden in the wall along side an empty patch of land.
Icicle Plate: Route 217, talk to the man inside the north-east house.
Fist Plate: Route 215, near the beginning of the route, past the cut trees.
Toxic Plate: In the Safari Zone, take the train to Area 1, then go the the most northeastern corner of the Safari Zone and search there.
Earth Plate: Oreburgh Gate, go to the basement and surf all the way to the left. It's somewhere there by a strength boulder.
Sky Plate: Pokemon League, under one of the statues just outside the exit of victory road.
Mind Plate: Solaceon Ruins, at the end among the stash of items.
Insect Plate: Eterna Forest, in the right wall just outside Eterna Old Chateau.
Spooky Plate: It's in Amity Square. That place where you walk with your cute pokemon. In one of the buildings.
Stone Plate: Mt. Coronet, when you reach the snowy area, look for an entrance that leads you into a cave with a waterfall. Climb the fall and it should be up there somewhere in a tiny cave.
Draco Plate: Behind the Dalgia/Palkia statue in Eterna City.
Dread Plate: Eterna Old Chateau, inside in the top left room.
Iron Plate: Iron Island, somewhere in the lower part of the cave.