This place stocks all the tea in China, and then some. More than 300 vendors occupy three floors, but most vendors sell the same tea, so find a seller with whom you have a rapport and sit down for a taste test. You can buy such famous teas as West Lake dragon well (longjing) tea, from nearby Hangzhou, and Wuyi red-robe tea, as well as the tea sets to serve them in.
Though the vendors encourage you to taste all their teas, as you should, they are not terribly pushy.
520 Zhongshan Xi Lu, Shanghai, 200051, China