Troves are the only original source for that material. Fine or Coarse.
You can always use the Crown Mimic Stones.
And, it is a trade-able material, like all Style Mats, so you could find some in Guild Stores
Fabulosity wrote: »The easiest way is to deconstruct weapons and armour that are in the thieve's guild style. I think Bahara's Curse is in this style and is common enough from thieves guild dailies/story quests.