If you find it anywhere, it is at least a couple of months old which means it shuold be fresh for a couple more. That being said, try the Westover Market on Washington Blvd in Arlington. It isn't far from the East Falls Church Metro. They literally have thousands of beers. Some you can't even find at Total Wine & More (Total Beverage to those living in the 90s).
http://westovermarket.com/2038472.html
Happy searches and here's to you finding your Punkin Ale!