Turtle Scale can be obtained by skinning Turtle from level 35 to 50 unless they are rare creeps. Turtle Scale can be used to craft a five piece Turtle Scale Set. There are many places to farm Turtle Scale which includes Thousand Needles and Tanaris, but the best place to farm Turtle Scale would have to be the swamp at the most south eastern of Swamp of Sorrow and farm the Sorrowmurk Snapjaw. Respawn rate is decent and they are quite packed together.
See below for the exact location to farm Turtle Scale in WoW.