Exotic Leather is the new leather introduced for the Mist of Pandaria Expansion and can be obtained by skinning most beast on Pandaria. If Sha-Touched Leather is obtained instead, a skilled leather worker may be able to convert 5 pieces of Sha-Touched Leather to an Exotic Leather. Beast at level 87 or higher will yield a higher chance to produce Exotic Leather. The best place to farm Exotic Leather would be Timeless Isle which is just of the coast of Jade Forest. The beast that you will be looking for would be the elite turtles called, Great Turtle. These Great Turtle is between level 87-90 and is not difficult to slay. Many players leave their turtle carcase from the daily quest, so they can be skinned. Great Turtles can be found to the west of Timeless Isle near the coast.
See below for maps of where to farm Exotic Leather in Wow.