My champ has one. One of my other characters found it as loot (no idea where) and put it in shared vault for him. His second one was crafted by my weaponsmith.
Probably your best bet is to scrounge the shards and ingots and ask in the Snowbourn Wildermore crafting areas if anyone can make you the 3rd age. Link the mats in the /say text so they know you're prepared and they just need to do a quick assemble.
Otherwise you have to grind level 85 humanoids and hope one drops, which would be the crafting instances.
Cener, Ingo, Rilibald, Hesred, Halras, Loringo
Arkenstone (ex-Elendilmir) - The Osgiliath Guard