The NAS-81 potion can always be found in 2 places.One is dropped by a Savant Guard who is with a group guarding the Savan't Tower in Arnika.
The second one is in the same area as where you find RFS-81.
This one is a hidden item so you'll need either a Ranger's scouting ability or the Detect Secrets spell active. It is located behind the hut that has part of it's roof caved in. If you're facing the opening of the hut where RFS-81 is found, it's the hut that is on the left of this one.
Spence