Yesterday I went to Ameyoko, which is a shopping district located in Tokyo between Ueno and Okachimachi station, to buy a pair of shoes. It is a place where there are lots of shops that sell clothes, shoes, foods, accessories, and so on. One can buy those commodities at a fairly low price, and at the end of a year, or Oomisoka, people flock to this area to buy foods to prepare for a new year.
It is summer now, but this shopping district was crowded and I saw several foreign people doing some shopping. Take a look at Tokyo Subway Route Map to see where Ueno station is. It is to the upper right of the center of the map.
You would find many shops that sell raw fish and seafood, which are fresh and cheap. For example, you can buy a king crab for less than 6,000yen (about 60 us dollars), if it is winter and fishermen are lucky enough to catch lots of it. However, you have to be careful to see if its quality is good or not.