I forget where I got it. Let me check the Dark Souls Wiki.
Edit: It's called the Covetous Gold Serpent Ring. It boosts item discovery by 200 points. Here is where to find it:
Sen's Fortress. Get to the area with the boulder directing device and move the lever so that it goes west (same way as proper way to go to continue upstairs). let the boulder roll once then move the lever North (empty space). Now go into the West path and follow it through to the end. You will find a ring on a corpse in a window and a way into a room where the firs MIMIC chest is.
Btw, that MIMIC chest. Kill it and you'll get a lightning spear from it. All mimic chests give guaranteed drops and don't respawn.
Edit 2: Sen's Fortress isn't available until you ring both bells.