Thick Leather

Post patch 4.0 I finally found mobs that consistently drop Thick Leather. Darkflight Soldiers and Raging Whelps outside New Kargath in Badlands. Mostly Thick Leather but with a few Rugged Leather in between.

Blackrock Depths is by far the best spot. You get ~55 Bloodhounds in a convenient sling ending up with about 50 thick leathers per ~9 minute run (mostly skinning time).

My sling: [url=http://www.wowhead.com/maps?data=1584:349774357770369768380765391766402765414770422776427790427805427819424840420854417875417899419916426937435951446959458963468976480974493963501951509937515922516902516875510858503842496829487821480800471782476763481741492733501718515716527704540693551669558654562631556617549600541596523611518627507642537582522572506570505623516642488566480578468603465621463638457658455677453695449712449726445753446741446772446788450811453825454846468852457860462871466883455897467899467910468924472590496568531570531600514615339778477852446800]Map Link

Grabs around 55 bloodhounds in an optimized no-backtrack sling. Then just run to the exit where there is a convenient vendor to sell the trash.

Each run averages at around 50 thick leather, 8 rugged leather, 50 runecloth, 160 dark iron residue and ~5-10g of vendor trash (although lots of tender wolf meat and red wolf meat + greens you might want to sell)

Did this as a lvl 85 Enhancement Shaman only using Earthbind (dwarves like to flee on low health) + Magma totem, so more dps could perhaps shave off a minute or so.

With the advent of AoE looting in patch 5.0.4, Blackrock Depths is unquestionably the best place to farm thick leather. If you are 85+ you can easily pull every Bloodhound on the upper floor. Start in Bael'gar's hall, then make a circuit of both circular tunnels. Be sure to grab the packs walking through the large cavern. Finish by running inside the supply area (don't forget to check the side rooms for dogs) and stop on High Interrogator Gerstahn. The entire pack will follow you the whole time and you can AoE them down in just a few seconds. On my poorly geared DK, a single run takes almost exactly 10 minutes to pull, kill, and skin. Better geared characters might be able to do it faster, although if you go any faster than that you will run into the hourly instance limit.

My average take per run after 2 hours of farming (10 runs) is as follows:

50 Thick Leather
20 Rugged Leather
1-2 Thick Hide
12-15 gold in vendor trash
50 Runecloth
2-3 greens

These results are with the Bountiful Bags guild perk.

I can't find a place that has a higher output than that, including Nyxondra's Broodlings.