Foxes in Vienna has a very limited selection, and two and a half hours down 95/64 is the Virginia Harp Center. If anyone knows of anything closer, I’d love to know, too!
The Virginia Harp Center in Philadelphia is about as close as the Midlothian location, at least if you’re in the District itself and want an excuse to make a day trip to Philadelphia (or if you don’t have a car). Otherwise you can also keep an eye on their web site. They have a lot of music when they come in for the MidAtlantic Harp Festival and sometimes a couple other times, usually somewhere in northern Virginia.