Pesach in DC
I second that suggestion. It's called Katz's Kosher Supermarket.
4860 Boiling Brook Pky.
The first time I did my Pesach shopping there I nearly fainted from happiness at the selection. I think you can take the metro out there and then it's a bit of a hike/bus ride to the grocery store. I'm not sure how long it took, but I definitely did it via public transportation. Try using the trip planner on wmata.com.