The pokewalker and the ruins of alph.
The color shards you can find in Pokemon Platinum are Red, Blue, Green, and Yellow Shards.
In HG/SS you trade Shards for Berries. Trade with Jugglers in Violet City and Fuchsia City.
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.
Use the move dive to go underwater to find shards in the seaweed.
There is no underground because you are in a different region. Only Sinnoh region has the underground, You have to find shards and heart scales in rocks. To do that you have to use rock smash.
find the guy who collects shards and trade shards for stones
There are an infinite amount of shards that you can get from wild Pokemon and limited ones you can find under the sea in rocks or above in rocks.
Mining and digging Underground.
Blue shards are found underneath the water while diving.
In Violet City or Fuschia City, there is a man with blond, spiked hair. He will trade you shards for berries (three berries for each shard). Each shard gives you a specific type of berry.
Ruins of Alph. If you smash the rocks then you will find a lot of shards and some fossils.
you can trade them with the guy in front of the Pokemon center in violet city and he gives you 3 different berries for each shard