Let it be known, there is not an "exact" location, but a general spawning area. Here are a couple tips at camping her, though it does require one to two friends.
My party setup:
Myself Blm/Rng
Wizardhermit Blm/Whm
Dasna Drg/War
I camped myself at G-7, inside the cave next to the Selbina Clay Tablet, using widescan to guide both my friends to the sheep.
At this location I was able to scan the entire area in which she could possibly spawn. Making this THE spot to sit, is that it sits in the middle of the spawn radius. We divided the area into two quadrants. One running seperate trains.
FYI, there are usually people camping Mary, especially these days, whereas, on Ramuh, the Horn is now selling for a solid million gil.
This setup worked out nicely, though the competition was slightly sore, stealing sheep from our trains.
About 14 stacks of sheep teeth later, I spot her on widescan, placing her on my tracking radar. I head right to her, telling my friends to head to F-5. No one was around.I cast poison to claim her anyhow. I knocked her in the head with my club's Brainshaker WS, then disengaged because everyone wanted to see her for themself.
Mary died a slow death resulting in poisoning, and a severe rupture to her skull, dropping some delicious milk and a nifty horn.
I took the milk and let the other two divide the profits on the horn since I already had one in my inventory.
Happy hunting:)