Tokyo Dining and Teppan Edo are the only places at Epcot you can make reservations for that serve sushi.
However, if you are on the dining plan, Teppan Edo (which serves a couple of types as appetizers) does not include the sushi at all (not even on the deluxe plan), and at Tokyo Dining the individual sushi menu isn't included in DDP. You can get sushi as part of a combo platter but you can't order some sushi rolls individually and have DDP cover those.
If you are going next month, you should probably try to make reservations as soon as you can.
While not at EPCOT, sushi is avalible at California Grille in the Contemporary prepared by a very highly regarded sushi chef (one of the top in the country).