on the route under mosdeep
The is a huge white rock, with dive spots round it, dive there, and you find an underwater cave. Go in and surface again and You find Sootopolis
all the shards for trading are found in the area west of Mossdeep in dive spots
You need to get the HM dive from Steven in Mossdeep City. Then you will have to beat the gym leader in Mossdeep City. Then you will be able to use the HM dive. And you find the dark spots in water while surfing. Use dive. And to resurface. Use dive again!
You can find different colored shards when you dive. When you see little dark spots on the bottom of the ocean they're items.
near mosdeep city, there are diving spots keep diving in them until you find a yellow shard then go to the house near the dive spots and talk to the man in tha house,
Red Shards can be found randomly in Dive Spots (the dark patches on the sea, in case you didn't know) on things such as rocks, or crators.
Steve
you first go to steven to get dive and dive dvie
Moosdeep City.
You must use the HM move, Dive, on the chain dark spots on Route 128. Then, go to the southern most area and there should be a opening in the wall.
The HM Dive is given by steven at his house in mossdeep city.
You need to dive somewhere then in dive in a different location hope this helps