I lived just outside of DC for 3 years. There is the Georgetown flea market every Sunday that had some neat things when I would go, but never found any signs really. And if you go to Leesburg, which is 45 mins or so away because of traffic, there are several cool antique stores. And if you go down route 15 near Leesburg, there are several antique stores. I actually bought my first sign at a store called Lucketts and got a ridiculous deal on it.