Arrowine in N Arlington on Old Lee Hwy carries D'Artagnag ventreche, pretty much the same thing as pancetta. The rounds are pre-wrapped and weigh about a pound. I can't recall if they also carry it bulk that can be sliced to order. They will also special order items for you, but I don't know if only from certain purveyors. You might ask about the Niman Ranch type because I think they carry some of their products.
Other places to try:
The Italian Store - N Arlington; Lee Hwy and Spout Run
Balducci's - McLean
Dean & DeLuca - DC/GTown, M Street
A. Litteri - Washington, DC, NE, I believe
Wegman's - I haven't been to one in some time, so I don't know if they carry pancetta; they seem to have a great selection of all other types of meats.
Best of luck -