My method for finding good sushi/sashimi in Japan is to go to the fresh fish section in the basement of a department store. (For those of you unfamiliar with Japan, the basements of department stores function as gourmet food shops and are a tourist-worthy sight--and source of free samples-- in themselves). There's almost always a little counter in the back where they serve up delectable variations on raw fish.

But I agree with one of the posters above. Even cheap sushi in Japan is quite wonderful.