More crystals

What are the different methods for acquiring more crystals?

Gathering and quest rewards are the only ones I know.

Botany and Mining are ways to supplement your crystal reserves. As REDace0 stated, some quests will also reward them.

Completing guild quest for your trade of choice usually yields a lot of crystals. I am a lvl 20 goldsmith and doing the quest from the goldsmith guild has provided me with hundreds of wind and fire crystals, more then enough to craft to my hearts content

I was given some in my Conjurer Quests, even at lower levels.