There is a large thrift store named Unique in Merrifield which has a lot of stuff. There's a Salvation Army store on Balls Ford Rd. in Manassas.
If you're willing to travel, I recommend the open air flea markets. There's a good one in Harper's Ferry, WV which runs from March thru November on weekends.
There's another large open air flea market in White Post that I think is every weekend, called Shen Valley Flea Market. The Shen Valley one seems a little more commercialized and it's reflected in the prices. I like the Harpers Ferry market better, but you'll have to wait until March for it to re-open.
You can also try the Strasburg Emporium, I stumbled on it online but have never been to it.
Also the DC Big Flea at the Dulles Expo Center was just this weekend.
I've been to the DC Big Flea once and the vendor's didn't seem interested in haggling at all, and I've seen better quality stuff at the flea markets for a fraction of what these vendors were asking.
http://www.harpersferryfleamkt.com/
http://doubletollgate.com/
http://thestrasburgemporium.com/
http://www.damorepromotions.com/Schedule/schedule2010.htm
(link says 2010 but I believe it's the 2012 schedule)