The lowest level mobs in Kael that have a possibly (that I know of) of dropping Thurg no drop quest items are the "Lieutenants" in the 4 way area (I.E. Lieutenant Vellos, Coldforge, Peldiyrem, Stormeye, Frostbite). Other than than drops are random from Arena and King Tormax area.
You will notice them as being the
Kromzek storm giants in the blue armor (no long beard). I will link you the P99 wiki entry and you can check the map locations for specific spawn points. Drop % is close to 2% from the Strom Giant Lieutenants.
https://wiki.project1999.com/Lieutenant_Vellos
This is a reference as I never seen armor drops from lower mobs than these.
If you like I did a video on this area to provide a frame of reference. If you want to take a look PM me or discord me and I can send you the link.
Sarleon#8503