Sold by Sandrice Gemheart on the ground floor in Froststone Crystal Caverns.
If someone bought the ToV Collector's Edition your guild hall may have the purple Froststone Crystal that will take you straight to the merchant's area.
Otherwise, you have to run through the Eastern Wastes (use IVU - invis to undead) and zone in. Once you get inside it's a dangerous run/fight unless you can get someone to give you both regular invis and IVU.