"Legions of the Dead" by L. Sprague de Camp and Lin Carter (contained in the book Conan the Swordsman)
"The Witch of the Mists" by L. Sprague de Camp and Lin Carter (contained in the book Conan of Aquilonia)
Marvel's Conan the Barbarian issues 252-260 by Roy Thomas
Savage Sword of Conan #39
King Conan #1
The Witchmen of Hyperborea (written by me) in Signs and Portents #9.
Unfortunately, even with those available sources, I doubt you will be able to glean much that was not used in The Road of Kings. The best bet might be to look up the recent Dark Horse comic, as that was not available to me when I wrote The Road of Kings (Conan #0 was the only issue published when I finished writing it).