U find lite + shaped dive spots u can find them everywhere but there isn't always somthing there there is a little patch on the ground where the item is,
They can't be bought. You can find them underwater, or attached to wild pokemon. Chinchou have yellow shards, Clamperl have blue shards, Relicanth have green shards and Corsola have red shards.
You can trade them for evolution stones eg. yellow shard = thunderstone
Yellow Shards and the other color shards are used for trading with NPCs for items or moves. For example in Pokemon Ruby, Sapphire and Emerald they are the main source of evolution stones.
Catch Clamperl, Chinchou and Relicanth most of these Pokemon hold shards
they come from the clamperl
The color shards you can find in Pokemon Platinum are Red, Blue, Green, and Yellow Shards.
You can get them underground.
Use the move dive to go underwater to find shards in the seaweed.
Blue shards are found underneath the water while diving.
Red shards are found underwater.
Green shards are found underneath the water (or in the sand on islands) while diving.
There are four shards in Pokemon Platinum- red, blue, green and yellow. They can all be found in the Underground.