Potency crystals come from paid events. You can win them from various games at Hollow Eve if you have a ticket, but they are also available for purchase from microtrans events (Duskruin, Ulf'Hara, Su Helmas, etc). There are a lot of these in circulation, and you might find someone from the fest willing to sell a few?

Potency crystals have a sibling known as infuser stones. Typically, potency crystals add features or increase the function of an item. Infuser stones will usually make the item usable more often through adding charges or possibly resetting the cooldown period.

https://elanthipedia.play.net/Item:Potency_crystal