Heavy Hide are much rarer than Heavy Leather can be skinned from beasts between level level 25 to 45. Beast at higher level will yield a better chance of obtaining Heavy Hide. A good instance to run for a good yield of Heavy Hide would be Razorfen Kraul. Skinning Agathelos the Raging, a level 35 Elite which is the last boss will almost definitely produce a Heavy Hide and the Greater Kraul Bat and Rotting Agam’ar may drop a few Heavy Hide as well. From a single run may produce 2 to 4 Heavy Hide. Keep in mind although this may be tedious and time consuming, you should be farming Heavy Leather at a constant rate.
See below for a map to farm Heavy Hide.