Hello:
Working on my newb armor quests for my Erudite Magician, and was dreading the exploit of finding a calcified ribcage for the Final Task weapon quest.
Anyways, all I had to do is keep killing the Skeletons (not Decaying Skeletons) that we're found on a fairly regular basis just south of the entrance to Erudin.
Mind you I did get many other Calcified Skeleton parts before finally getting a Calcified Ribcage drop.
For my lvl10 Magician, the Skeletons were light blue con, and a few green con.
Exact location where I stood to intercept the Skeletons on a regular basis was:
+1975, +134 which is just south of the entrance to Erudin as mentioned above.
Hope this helps someone else.