Can also find great designer label clothes at thrift stores. I shop Savers all the time and have picked up all kinds of great finds (Eg. True Religion, Diesel, Guess Premium, 7 for all Mankind, Miss Me - each pair of jeans for under $15 ; also lots of tops and dresses for under $5 - BKE, International Concepts, Apt 9, Maurices, Heart and Soul, Free People, Alfani, etc.)
You just have to make sure there is nothing seriously wrong with it, some of them do have issues like stains/dirt, pulled threads, missing buttons, or split seams. Some of these are an easy fix though if you have a little sewing skill. Although I have found plenty of clothes in new condition, and even with the store tags on them. So there is the chance you find something like that as well.
Some people actually shop these thrift stores during sales and then resell their finds on ebay to supplement their living.