Thick Hide can be obtained from creatures that drops Thick Leather and comes from beasts between level 30 to 55. Higher level creatures will yield a better chance to drop Heavy Hide. Tanaris and Un’Goro Crater is full of creatures that can be skinned so both places would be good to farm Thick Hide.
See below for maps for areas to farm Thick Hide.
Beasts to look for at Tanaris:
Blisterpaw Hyena, Glasshide Basilisk and Duneclaw Lasher