Thanks for you answers.
I don't wish to buy online, thanks anyway for the address, could be useful another time
The first places I thought of were Sogo and Mitsukoshi, I think it can be a correct option. Though, I more especially seeking a specialized tea shop, maybe like Wistaria house ? I will check that
The idea of asking around japanese restaurant is very good, I note that down
By the way, maybe you know if 玉露 is the correct spelling for Gyokuro ?