Wintersbite can only be found in the Alterac Mountains on Eastern Kingdoms, (not to be confused with Alterac Valley) specifically in the Ruins of Alterac region. A skill level of 195 in Herbalism is required to gather this herb. It's a grind to harvest, but the rewards are plentiful. Wintersbite can be a lucrative selling item in the Auction House because of its rarity and somewhat painstaking and time consuming collection-effort.