If you would like to taste "Macha" and experience "Sado (the way of tea)", I recommend Jidaiya in Asakusa (http://www.jidaiya.biz/index-e.html). They provide Sado experience with the reservation in advance. You don't have to prepare any materials or equipment. According to the website, they will explain the basic things in English, the the lecture won't be fully in English.
If you have a chance to be in a supermarket, every supermarket has several kinds of green tea.
If you are looking for something nicer, Ippodo in Marunouchi (near Tokyo Station) or Ocharaka in Nihonbashi might be good places. They have nice packaging, too.
Another shop you can easily find is LUPICIA. It's in most shopping malls such as Solamachi in Tokyo Sky Tree Town. They mainly focus on English tea and blend teas but they have a good selection of Japanese tea as well.
Some other shops are listed here, too: