Core Leather is a rare leather that can only be found by skinning high level creatures in the Molten Core or Firelands. Core Leather are use to craft high level plates, mails and leather armours. It can also be turned into the Thorium Brotherhood for Reputation points. The best place to farm Core Leather is The Firelands, from west to the south of The Molten Fields to slay Ancient Core Hound.
See below for the exact area to farm Core Leather in WoW.