The best place to harvest those skins is not in the mines or in the temple, but in Frontier Mountains. The goblins in The mines of Nurga and in the Temple of Droga do occasionally drop green skins, but not nearly as often as the goblins in FT. You'll get faction hits, but just a handful of skins.
On the other hand, in Frontier Mountains, you can get a good amount of skins by killing the goblins in 4, 5 and 6. There are a bunch of goblins waiting to be killed at the entrance of the mines of Nurga and the temple of Droga. Also, there are a couple tunnels located south west of the Giant Fort, with plenty of goblins, plus a couple Iksar slaves.
Easy kills, faction hits and lots of green goblin skins.