If you're looking for instant gratification, head over to
Granddad's Hobby Shop. They're literally a block outside the Beltway off Braddock Road, so it's no more of a hike than to the Pearl Art in Alexandria (assuming that's the one you were going to, I used to work there years ago!). Despite it's relatively small size, Granddad's is also just about the most well stocked model & train store I've ever step foot into. I think you'll find it modeling nirvana.
If they don't have the plastic you are looking for, and you also want to pick up some other items, I recommend the
Great Model's Web Store, which is a brick & mortar store out in Utah that has a long established web operation on top of it. I've ordered from them several times and they have great prices, they carefully pack your order and are good about communicating with you. The only drawback is they can sometimes be a little slow, taking a more standard 2-3 weeks (sometimes longer) to fulfill orders than the high-speed 1 week or so some other web stores have spoiled us with.
Edit: For other local places, I want to say there was a Hobby Lobby or some other model store in the Pentagon City Underground mall (there was also a fairly decent comic shop there too,
IIRC). As anachronistic as this sounds, also check your phone book, you might find some place that's opened nearby in the two years since I lived in the region (and the five since I lived in Arlington).
