I get mine with Artisan Spectacles. The rate is relatively low...I usually get 2-4 for every 25 that I desynth. You can also get Fieldcraft Demimateria III this way, but I think this rate is much lower than it used to be - I get about 1 for every 50 desynths of those. I have better success with buying Bridesmaid Sandals in Uldah for the fieldcraft ones.

I know some people like to get their Artisan Spectacles with seals, but 5000 each is pretty steep for where I am now so I usually farm hippogryph skins to make the straps and trade them in, which I have found the easiest.

Happy crafting!