Here's a recent thread on this topic: http://youlookfab.com/welookfa.....replies=12
Greenglove is one of our resident expert resalers, as are Sarbear, Antje, and Brianna.
The general wisdom is that you have to go often, look hard, and know what you're after. I have found many wonderful items in thrift and consignment shops over the years. For me, they present a more interesting hunt.
Years ago, long before Gap and other popular retailers carried maternity, when the only choices were horrible, cheap polyesters or incredibly pricey maternity boutiques, I outfitted my entire nine months of pregnancy from specialty resale shops.
Years before that, I spent many hours during college in a vast thrift store run by a religious mission, with gospel music blasting from every speaker. In those days, I was more into vintage items, and I found some incredible treasures: raw silk ladies' suits straight out of Madmen, cashmere coats, heavily beaded cardigans from the 50s. I don't think I ever paid more than $8 for anything.
Nowadays, I'm more likely to spend time in a local consignment shop that is quite picky about what they take, because my time is more limited and my budget slightly more forgiving than when I was in college. But I still love the thrill of the secondhand hunt...