Devilsaur Leather can be found by skinning Slavermaw in Searing Gorge and Devilsaurs, Ironhide Devilsaurs, Tyrant Devilsaurs, and King Mosh in Un'Goro Crater.
As an ingredient
Blacksmithing
Leatherworking
Patch changes
Patch 6.0.2 (2014-10-14): Stacks to 200, up from 20.