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.
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In HG/SS you trade Shards for Berries. Trade with Jugglers in Violet City and Fuchsia City.
The color shards you can find in Pokemon Platinum are Red, Blue, Green, and Yellow Shards.
You can get them using the dowsing MCHN on the Pokewalker or you have a chance of getting them by using Rock Smash on any rock (You can get red, blue, green and yellow shards this way as well as heart scales, revives, max ethers, and fossils).
You cannot buy but you can teach your pokemon new moves in exchange of shards.
There is no best Pokemon on Pokemon HeartGold.