You get them when finishing some quests. Also devs give some for free when servers crash. And in festive holidays. Otherwise you have to buy them.
You can get crystals from: Quests Devs being generous Buying them Quests- If you have not completed them all yet you can still get them from them. Devs- Almost every holiday or when the devs feel generous they give 3 free crystals Buying- You can buy 5 crystals for $5 How to get money for crystals: Save your change Get an iTunes gift card as a present (its almost Christmas time) Ask your parents (if your under 18) Save change- You can save your change that you get, find, or given to buy an iTunes gift card. Cheapest one is $15 and they don't have tax since its a gift card.