Several good places are:
Lebanese Butcher in Falls Church, which has the best of the lot.
Arax on Lee Hwy in Arlington. It's just a small sandwich place with a couple of tables. I suspect if you want just felafel without the pita and trimmings you can get it.
Jerusalem just off rte 7 in Bailey's Crossroads. Used to be second to Lebanese Butcher, but the place has been slipping lately.
Layalina on Wilson Blvd in Arlington has an acceptable version. At lunch a sandwich comes with pretty good fries. It's been about a year since I've had it, though.
Lebanese Taverna has it of course. Although I don't care for it in their restaurants, the one from the Lebanese Taverna market is acceptable. It's near where Lee Hwy and Old Lee Hwy split.