ive not found one yet.. the drop rate is so incredibly low, ive not been able to narrow it down, even if i am in a good spot it still feels so completely random to me that i dont notice it as a good spot. i spend lots of time farming cooking mats to try and get the cooking rewards,, so ive been all over the continent.
edit: i bet if we spend too much time trying to farm these it will be wasted in the next build when they either remove the soulbound part.. or increase/change drop locations. and then say "ooops, our mistake, we didnt mean for that to happen". personally as eager as we may be to get on with doing the cool profession stuff, i think waiting at least to see what changes come in the next build is the better option.
Before the idiotic BoP change I was getting 2 per day (while doing dailies - be KW, TS or even normal questing in dread wastes and Vot4W), after that, have it to see 1 drop.
Worst thing... I had 15 stored in a lvl 1 bank toon that I was planning to use later on my tailor.