It can be obtained from a wastelander merchant in The Oasis Merchant random encounter. He will suddenly die, seemingly drug induced, allowing you to loot the note from his corpse which in turn marks Oasis on your Pip-Boy map.
By tomorrow I should reach the green mountain. The people there may be weirdos, but hey, they trade great stuff for the weapons and ammunition, so who am I to complain?
Last time I was there, they told me that someone named the Great One wanted to speak to me, and that I should drink some of their homemade hooch and join them in some kind of ceremony. I'll probably take them up on it this time - I haven't had a good drink in a while and I'm a day ahead of schedule.