Refer to each item
Earn with demand of land and hand equipment
Stormsap is a material added in patch 4.3.
It is also a material of the new land and hand equipment added in the same patch 4.3.
It will be used in all parts, and if you arrange all equipment of land and hand, nearly 100 pieces of Stormsap will be needed.
Namazu beast tribe quests
The easiest thing is to replace the Namazu Koban with the Stormsap.
Since exchange rate is 1 piece per piece, you can receive 3 pieces a day.
Desynthesis of Rabanastre equipment
It seems that Stormsap appears even by Desynthesis equipment available in Ravanasta.
Yellow scrip exchange equipment
Stormsap can be obtained even by Desynthesis equipment of IL 330 or more that can be exchanged in Yellow scrip.
Personally I have not tried the IL330, but I confirm that it appears from the ring of IL 350 to a crafter or a gatherer.
I think that you can think about whether to try this method from your Desynthesis class or level.