Head to the Gallows where you can turn in a quest to Record Keeper Teryn. Now speak to Solivitus to obtain the quest 'Herbalist's Tasks' . The Spider's Silk Gland can be found on any of the large spiders you fight in the Abandoned House or Ruined Passage. If you already went there during previous quests and searched thoroughly then you already have this item. The Dragon Fang is obtained from defeating the Mature Dragon at the end of The Bone Pit quest. To gain the Pure Ironbark head over to Master Ilen in Sundermount.
He informs you that he has nothing left in stock but divulges information as to its whereabouts. Leave Sundermount and head to the Ironwood Clearing where Darkspawn lurk. Follow the path and turn west, defeating the Darkspawn, then looting the rubble . Head north and defeat the slew of Darkspawn until an Ogre appears. Once the Ogre has fallen you may find the pure ironbark in the north east corner with a corpse . With all items in hand return to Solivitus to complete the quest.