If you have a car, you can go to the DC Big Flea. It’s at the Dulles Expo & Convention Center from 9am-6pm both Saturday and Sunday. It’s $6 to get in, but with over 1,000 vendors you will probably find something fairly awesome to get compliments on. I tried to use the trip planner on wmata.com to see if I had any options to get out there, but there doesn’t appear to be. Using Zipcar to get there would probably be pricey as well if you plan to do some serious shopping.
4368 Chantilly Center