How to get elemental crystals?

In my world how you cast spells is by focusing spirit energy in an elemental crystal. How powerful or type of spell you use, is how big or what type it is. What I'm having trouble is how to acquire/ mine a couple of them.

It somewhat ideas for most of them:

Fire Crystals you get from ashes from campfires and forest fires.

Water Crystals you get at the bottom of lakes, seas, and oceans.

Earth Crystals you get it from mining them deep in the earth.

Lightning Crystals you get from where Lightning strikes something like trees or fields.

What I'm having trouble with is where you would get things like Wind Crystals or Dream Crystals.