Is there a set place where I can find a de-enchantable necklace of Poison Immunity or Poison Resistance? Its the last enchantment I need before I can start creating my end-game equipment.

Wait, it does exist, doesn't it? Is it just random loot? Can it be bought?

I refer you to this thread: http://www.gamesas.com/topic/1322643-muffle-resist-poison-disenchanting

Good luck!

You may find resist poison enchantments, yes.