This Thick Leather farming guide shows the best places to farm Thick Leather in Classic WoW. It also works for TBC and WotLK, since there were no major changes to the old zones.
Thick Leather
Dustwallow Marsh
Stonemaul Ruins in Dustwallow Marsh is one the best place to farm Thick Leather because the mobs often drop two pieces of leather instead of one. Move between the Den of Flame and Stonemaul Ruins.
If they don't respawn fast enough for you, you can do one lap around Onyxia's lair until you wait for respawns.
Mob Level range: 40-44
Un'goro
Kill Ravasaurs in the marked area. They have around an 75% chance to drop Thick Leather, and the fast respawn rate means you won't have to wait between pulls.
If you get a 
A Mangled Journal, just accept the quest and complete it. Otherwise the item will keep dropping and you will have to delete it.
Mob Level range: 48-51
Searing Gorge
You can find many Incendosaur inside the marked cave in Searing Gorge. They respawn continuously, so you can farm them as long as you want.
They can be a bit tricky to find. Once you enter the cave, take the first right and keep going until you reach them.
Mob Level range: 47-49
Stranglethorn Vale
Ironjaw Basilisks and Thrashtail Basilisks inside the cave in Stranglethorn Vale are a good alternative if you don't want to farm the other places. But, I would only recommend this place if you are a low-level player. High-level characters will kill these too fast and have to wait for respawns.Mob Level range: 41-44
