Where can I get crystal pieces?

A simple question.
I’m playing on Randomizer, and managed to snag me an Arceus with a ball I got off the table in the Abyssal Cult. I was told the Crystal Pieces aren’t randomized so you don’t get one in the first town, but…
When I inspected the Infernal Cult Leader’s body, I got a Calcium, not a Crystal Piece. I was going to use that to improve my Arceus, but now I can’t, and according to the last 12 hours of my life you can’t nab them off a Pokémon either.

In short, how can I get Crystal Pieces? Can I get them from breaking rocks like other items, or from randomized shops?