It can be obtained from a wastelander merchant in a 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.