The thieves guild didn't give you a location? You can't just go buy a pearl at a jeweler's shop and hand it over like in Morrowind or Oblivion (now THAT was something that needed fixing). If someone tells you to find an object at the bottom of the Dungeon of Hawthorne, you have to find the object at the bottom of the Dungeon of Hawthorne; no substitutes will be accepted. If the item isn't tagged "quest object", it doesn't exist, so far as the quest giver is concerned.
Now, realize that if you just can't finish this quest, that's okay. Give it as good a go as you can until your time runs out if that's what you want to do, but failure is acceptable in this game. Just get another quest, whether from the same guy when your time runs out, or from another thieves' guild in another town.