This item only drops from Savanna Rarabs in Sarutabaruta. I spent over an hour killing random Rarabs in both East with no drop. I then decided to go and see if I could get this to drop from those surrounding the Star Tree. I figured this since the Meteorite for the Star Struck quest is only dropped by Crawlers directly around the Star Tree. On the way to the Star Tree I killed any Rarabs I saw, in which I had no luck. When I got to the Star Tree, the Torn Epistle dropped from the very first Rarab I defeated, I kid you not.
The way the quest is activated is in the same manner the quest involving the Damp Scroll is in Norg. You speak to the NPC carrying this in order to open this quest. The NPC will say something different then if you aren't carrying the Epistle and trying to open the quest.
My suggestion to anyone wondering is to stick to defeating the Rarabs around the Star Tree in West Sarutabaruta.
WS i got mine, didnt even know what its for, killed a billion of these things and never got one so keeping until i know what it is. It says its a love letter addressd to a man, someone must of ment for me to find it XD