You can find this shop in The Warrens. You must have completed the quest, The Queen of Thieves, up to a point where an NPC tells you where it is exactly. Even though, this general store is found in Great Kourend. You do not need to visit it to complete the quest, The Client of Kourend. You can only visit this store after you have done this quest.  You will need to visit this general store to complete the Queen of Thieves quest. This quest will give you one of the 5 teleport pages you are going to need to complete the memoirs and unlock the teleport.