The sushi in Japan is very good. There is a Tempura bar that is walk up only on the 2nd level between the Matsu No Ma Lounge and Mitsukoshi Tempanyaki. They have really good sushi, made right in front of you.
At Downtown Disney Westside Wolfgang Puck Cafe has a Sushi bar, and I have heard it is good.
Kimono's at the Swan is a sushi bar... they also have karaoke, so if you want to sing and eat sushi, this is the place to be!!!
California Grill has sushi, too, but if you haven't made advance PS you can't walk up. So if this is your choice, make sure to get those PS.