As a Clothing Enchantment:
Newer Deathblock enchantment with a Roman numeral reduces negative energy damage by 3 percent per rank. Unfortunately, this reduction also affects the healing of Pale Master wizards in undead form. .
Other sources: 
- Green Steel accessories
- Yellow augment
- Cannith Crafting
- Bug: Applying a "Deathbock Shard (Unbound)" to a heavy armor will make the armor BtA.
- Embraced the Void, in the Pale Master enhancement tree (needs level 20)