Crystallized Shadow

The Deathbringer Revenants in Frostmourne Cave (Dragonblight) also drop these, if you're looking for a place earlier in the game. Don't know the drop rate, but I got three in about 20 minutes. And the respawn rate is good, I never stopped fighting.

Just to pool information from many different pages relating to this item. This item can be obtained from 5 different sources:

1. 
   
2. The Auction House
   
   • However, these may be very expensive depending on your server.
   
   
   
3. 
   
   
4. If you have Mining:
   
   • 
     
   • [object=189980] (mining level 400)
   • 
     
   • [object=189981] (mining level 425)
   • 
     
   • These two nodes appear to be most abundant in Icecrown, Northrend.
   • 
     
   • NOTE: if you see the nodes underground by [zone=4812], they are accessible by jumping down into a long, vertical tunnel around the middle of the outer-portion of the Citadel. It will dismount you if you fly down it. Don't fret; there is a portal next to a human child, within the cave below, that will port you back above ground.
   
   
   
5. 
   
   
6. Regular mob farming:
   
   • 
     
   • [npc=27382] located in Dragonblight, Northrend.
   • 
     
   • [npc=30842] located in Wintergrasp, Northrend.
   • 
     
   • [npc=30872] located in Wintergrasp, Northrend (only visible when your faction controls the zone).
   • 
     
   
   
   
7. 
   
   
8. If you have Frozen Orb:
   
   • 
     
   • You can purchase one Eternal Shadow from [npc=40160] in Dalaran, in exchange for one Frozen Orb, which can be obtained in 3 ways:
     
     1. 
        
     2. Auction House: may be expensive.
     3. 
        
     4. Purchasing one from: [npc=31580](A) or [npc=31582](H) with 75g.
     5. 
        
     6. Killing and looting the last boss of any heroic, Northrend dungeon.
     7. 
        
     
     
     
   • 
     
   
   
   
9. 
   
   
10. If you have Alchemy:
    
    • 
      
    • You can Transmute: Eternal Earth to Shadow (Must have alchemy level 405 and at least be character-level 45).
      
      • 
        
      • This pattern is "Discovered" according to Wowhead.
      • 
        
      • Eternal Earth = 10 Crystallized Earth.
      • 
        
      • Note: According to owlmanatt, this transmute's "cooldown is shared with the Cataclysm transmute CD."
      • 
        
      
      
    • 
      
      
    • You can Transmute: Eternal Life to Shadow (Must have alchemy level 405 and at least be character-level 45).
      
      • 
        
      • This pattern is "Discovered" according to Wowhead.
      • 
        
      • Eternal Life = 10 Crystallized Life.
      • 
        
      
      
    • 
      
    
    
    
    
    

    ---

    
    EDIT: Updated information to reflect the removal of Justice Points in WoD and organized the post so it didn't take up so much room.

A great place to farm these is in wintergrasp when your faction controls the zone.

The little mobs there called 'wondering shadow' have a chance to drop them, feels pretty low to me, maybe 10%, BUT, when your faction has the zone you can see bigger mobs called 'Shadow Revenant' they have a good chance to drop them, up to 3 at a time.

These guys are big and have about 25k health, theyre still no trouble to solo, but life will be made difficult if you try to go around peak server time and have trouble from the opposing faction.