Thrift Stores?

I'm going to a long time friend's wedding this summer and need a new dress. Dress code is 'as formal as you're comfortable with' and before I drop a lot of money on a dress from a website I'd love to go back to my broke girl roots and check out thrift stores to see if there are any treasures. Any people recommend? Must be more economical than the $150 dress I have my eye on online or what's the point.