MMM i think you´re talking about
Ameyoko-cho wich is just across the street as you exit Ueno Station. It looks kinda like a flea market but they are mainly selling food items. Its a nice walk anyway because its so different from the rest of Tokyo, so i´d recommend to go there! There is a nice Shop somewhere on the other end of Ameyoko-cho wich sells rice with raw fish for a good price.