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The Rusted Coin is an item that temporarily boosts your Item Discovery, and is worth using since it stacks with other boosts you may already have. However, for late game farming, you'll either want to be sure to upgrade the weapon so it can actually damage enemies, or wield it in the off-hand instead of the Shield of Want, while using your main weapon to kill enemies. Simply wielding the weapon will raise your Item Discovery stat, making it great for farming. The Crystal Sage's Rapier is a special weapon transposed from the Soul of the Crystal Sage, which can be gained early on in the game. Because of this, you'll only want to equip this when you have a farming spot close to a bonfire - or at least pack lots of Estus.
This helm boosts your Item Discovery Rate - but at the cost of constantly draining health. Every Mimic you meet has a chance to drop it, but the last Mimic left in the game should drop it regardless - if you haven't found it before then.
The Symbol of Avarice is a funny looking Mimic Hat that is dropped from killing a random Mimic in the game. Dodge it or kill it, and look around its lair for a body holding the shield. Take a detour down the bridge before High Lord Wolnir to enter the Smouldering Lake, and head left to where a Giant Sand Wurm has made its home. The Shield of Want is a special magical shield that allows you to obtain more souls from enemies, much like the Covetous Silver Serpent Ring - though it's located about halfway through the game.