@murphyslaw - D.C. Has some good places for American sours. Churchkey (near Logan Circle) usually has some good ones on tap and in bottles, and Right Proper Brewing has their own as well as guest taps (Pub in Shaw and tasting room in Brookland). I've heard Dacha has them as well, though never been.
In Baltimore, you have Stillwater Artisinal Ales, as well as two of my favorite pubs, Monks in Fells Point and Of Love and Regret on Brewery Hill (just past the Natty Bo brewery).
Head a bit northwest to Ellicott City, and there's a phenomenal Farm brewery called Manor Hill that has a sour blonde and 1 or two others. Same ownership as Victoria Gastropub in Columbia which ahas a few offerings in addition.