Thai//Vietnamese/Asian Grocery in DC or Arlington
The grocery store in Eden Center should have stuff in theory, though I don't know for sure. It's a 20 minute walk from the metro station, which is just barely "metro-accessible." A relatively pleasant walk, though. And then you can combine the trip with banh mi or other Vietnamese eating.
Falls Church, VA, USA, Falls Church, VA
are you looking for ready made thai tea or a tea mix?
for thai tea mix, there is a thai grocery store next door to bangkok 54 on columbia pike in arlington. it's related to the restaurant so they also have food packed to go in their refrigerated section. they definitely sell the tea mix there because i've bought it there before and they probably also sell tapioca balls, too. i don't think the grocery store sells the tea ready made but i'm sure you can get it to go from the restaurant. it's $2.50, i believe.