You can go to Tsukiji Fish Market. Lots of sushi and other delicacies.
My favorite conveyor belt sushi restaurant is "Nemuro Hanamaru" near Tokyo Station. I recommend this place every time I asked by someone (including foreign tourists) and all of them said the sushi there was great!
The restaurant is originally from Hokkaido, where said to be the source of the freshest sushi in Japan, and they deliver fish from Hokkaido to Tokyo.
Budget is about ¥3,000 - ¥4,000/person.
You can go to conveyor -belt sushi in Tokyo.Espechially, ”UOKATSU" in UENO is fresh seafoods and there are many types.So you can enjoy good taste sushi.