These skulls drop from the wardens in Harloeg in the Lone Lands, especially near the ruins of Nindor. One dropped for me from a Noxious Bog-Warden there.
I have been searching for these for several days now and have not found 1 of them so far I have been in the barrow-downs by Bree and in the lone lands by Haragmar without success If anyone has found one recently, could you please let me know where.