I was also able to harvest these at the harvesting points on the NW side of West Sarabaruta, starting west of the outpost, working my way around the ridge southward. I'd consider it rare, as I only harvested one the entire time I was there (also got herb seeds, vegetable seeds, moku grass, saruta cotton...)
what server are you on? I'm on Sylph, and none of the merchants in Selbina sell these. Not even the Galka that you refer to. Maybe Windurst has to be the dominant city? Right now San d'Oria is
I grew these in a porcelain pot from a tree sapling. I planted it on Windsday (I was trying for Wind Ore) and fed it one crystal on Windsday but I missed the feeding window for the other crystal. Then I harvested it on Windsday.
Porcelain pot + Tree Sapling (Windsday) + Wind Crystal (Windsday) + no crystal = 26 clumps Windurstian Tea leaves.