In Rockville, at Wintergreen Plaza, there is a Chinese owned store called Ten-Ren. Tons of loose leaf tea, some super expensive..but others affordable..they also make packaged tea that they sell in the Chinese grocery stores..I am a serious green tea drinker...(loose leaf at home, tea bags at work).5-6 cups a day, and I buy loose leaf Japanese tea at either the Japanese grocers...Maruichi in Rockville, at the Talbott Shopping center, or in the Chinese ( Kam Sam in Rockville or Korean ( Lotte in Germantown, Grand Mart in Gaithersburg)grocery stores...
Teaism sells decent loose green tea. Better than Whole Foods, but still expensive.
I get my loose gunpowder tea at Swing's.
You might want to try Russian Gourmet; there's one in Rockville. Not sure about green, but they have good pekoe and black teas.