Dragon Flight

Free Routes:

Lustrous Scaled Hide - The Waking Shore
Dense Hide - Ohn'ahran Plains

Battle For Azeroth

Free Routes:

Coarse Leather - Drustvar
Dredged Leather - Uldum
Dredged Leather - Vale of Eternal Blossoms

Legion

Free Routes:

Stonehide Leather - Azsuna
Stormscale - Highmountain

Warlords of Draenor

Free Routes:

Raw Beast Hide - Shadowmoon Valley

Mists of Pandaria

Free Routes:

Exotic Leather - Isle of Giants

The Burning Crusade

Free Routes:

CrystalInfusedLeather - Blades Edge Mountains
CobraScales - Nagrand
Knothide Leather - Netherstorm
NetherDragonScales - Shadowmoon Valley
WindScales - Shadowmoon Valley

Vanilla

Free Routes:

Heavy and Thick Leather - Feralas
Light and Medium Leather - Wetlands
WarbearLeather - Winterspring