Question

Where do I get a Comet Shard in Pokemon Black 2?

Answer 1

Non-repeatable
- Join Avenue (reward)
- Dragonspiral Tower (Northeast corner of 6F)
- Dragonspiral Tower (Hidden item in northeast corner of 7F)

Repeatable
- Wild Clefairy, Clefable, Lunatone, Solrock (1% in dark grass)
- Rugged Mountain (Dream World)

Comet Shard (Bulbapedia)

Answer 2

You can find it in the Giant Chasm.
Hope I helped!