When someone donates clothes to a thrift store or consignment shop, the store usually stocks the items to the exact same shop to which the clothes were donated.
If you're an avid thrift-store goer and on the hunt for swankier-than-usual loot, visit a store in an upscale area. Since the residents donating the items are wealthy, the stuff they donate is usually higher quality -- you might even score some designer brands you wouldn't find anywhere else!