All kinds of leathers(exept hides, and core leathers) can be found by 2 differents ways(a chest isnt a way xD):
1-Upgrading a low lvl leather(Ligth->Medium->Heavy->Thick->Rugged)
2-Skinning
I remember... When i was in arathi highlands, killing raptors, they usually drops medium leather(they also drop raptor hides, or heavy leathers, but in a inferior amount)
Best way: Buy at Ah, if u have an Alt char at lvl 60(i guess the main is a lvl 60,too), u can buy the leathers from Ah.