Well I have been trying all the mobs that cast myst spells and so far they are no better then earth eles in shame so maybe Im wrong about the drop rate and I just happened to have some left in my pack from an earlier attempt at useing fertile dirt to plant with.
With it being only 2 vendors and they(on atlantic) are mixed timed its kind of tough to "prime" them up but once done I could get enough to last a long while. 1800 fertile dirt please heheh.
This whole thing came to me when I saw a vendor in luna with a 60k deed of it and was so shocked that it existed I just had to find out how it was done. Im guessing it was the tel mur vendors.