Fel Scales got introduced for the Burning Crusade Expansion, as it can only be skinned from basilisks or ravagers in the Outlands. The chance of skinning Fel Scales from Outland creature is about 20% or 1 in 5 creatures skinned. The best place to farm Fel Scales is by slaying Marshrock Stomper and Marshrock Threshalisk right above The Dire Mire, Zangarmarsh. The basilisks are dense and respawns quickly, just be careful with the cliff edge.
See below for the location to farm Fel Scales in WoW.