I’ve been trying to find a high quality crystal for over a week to complete my collection.. am I killing things in the wrong location? I’ve been mainly focusing on the hidden monastery. Any advice would help. Thanks!

You can craft up to Unique Crystals using the Alchemy in your Camp !
You can also fuse Crystals to get Epic or Mystical grade crystals.

You can also buy low quality crystals from the merchant in any town.