So, Crystal Aron was an event about two years ago, so the only way
to get one now is to either buy an egg at the Event
Shop, buy/trade with someone, or breed them.
If you want to go the route of the event shop, the currency there
is Event Points, of which you can buy with nuggets/open mystery
boxes/(According to the wiki) completing tasks.
Crystal Aron is Uncommon, so it'll cost 2000 EP to get an egg from
the event shop. (Prices depend on the rarity of each event)
If you're wanting to breed them/shiny hunt them, there's only so
many eggs of the event you'll get. If you have one normal Aron and
one Crystal Aron, the chances of getting a Crystal is lower than it
would be with two Crystal Aron.
Here's the actual thing about it in the Wiki