Well,there are a couple ways to get shards in Black or White 2. The main 3 ways are these:
1. Go to rebirth mountain and go in the puffing spots. You can also use your hidden item detectro to find hidden shards in rebirth mountain.
2.Go to PWT and lose, your constalation prize will be a shard.
3. There is a finding shards funfest mission so once you collect them you keep them. They are the shiny spaces.
This is mainly it. Unless you want to look for shards in other caves.