Supposedly, there is a bind point in the Frontier Mountain zone according to Eqatlas.com. And the best place to fight Sarnaks from between the lvls of 20 and 30 is at the Sarnak Fortress in The Lake of Ill Omen. Find a group either outside the Fort Entrance (for lower 20's) or go inside and find a Platform group (for upper 20's).
I would imagine you could get a bind in the Elf outpost in Firiona Vie and the Sarnaks start at lvl 1 or 2 next to Cabilis city entrance and go up to about lvl 30 with one MoB in the fort about lvl 35 in the Lake of Ill Omen zone right next to Firione Vie. There are also some high lvl ones in the Overthere zone.