The short note can be found by inserting 100 numulites into one of the stone chests inside the House on the Hill on Fossil Island. There is a chance nothing will happen or a spike shooting out and stabbing the player for 9 damage, wasting the numulites.
This note can be added into the Fossil island note book.
For an exhaustive list of all known sources for this item, see here
Is this chemical warfare? Observations recorded today included a bizarre sight - a spore clump used a weapon of sorts which blinded and paralysed. I must be careful.