Sebilisian messengers can be found roaming all over the eastern and southern parts of the Field of Scale as they wander to and from prominent locations such as Cabilis, Kaesora, and Kurn's Tower.
loc -2400,+660. Sit on the south wall just a tiny bit west of the EJ zone and you will have more messengers than you would ever want. I killed 12 there just wandering by in the space of 15-20 min. Got 5 Sarnak Recovery scrolls, 6 non quest scrolls and the last was the Raising Cabilis scroll.
It spawns around the emerald jungle zone and they walk along the south edge, I have seen 3 at once. As soon as I killed one there would soon be more, within a couple of minutes.