Got pretty good Dalat tea at Ben Thanh and 500g of Thai Nguyen tea at Cholon, as well as some Thai Nguyen and Vietnamese TGY at Phuc Long, a chain of local tea/coffee stores. I also got something special from the people at Hatvala in Saigon, where I tried a lot of their teas over six hours. I really enjoyed their black teas, even though I prefer greens and oolongs. I was less impressed by their oolongs, but I was drinking rice wine from the far north of Vietnam and couldn't taste the oolong as well because of the wine.
I brought home Tiger Monkey green tea from Lung Phin. This is a high mountain wild green tea that is hand processed by the Hmong; pretty special stuff. The Hatvala team really go out of their way to find the rarest of teas in Vietnam. There are apparently other places to find the good stuff in Saigon, but this was the most readily accessible online and they know their stuff. Definitely worth a visit if you're in Saigon as they're very friendly and accommodating, and have a lovely tea room.
Hatvala do ship tea, but you have to e-mail them if you're curious about making a purchase. They don't have a cart on the site yet.