Some of the places you can find Sushi are at Teppan Edo and Tokyo Dining both Japan in Epcot. Also in Japan, for more of a counter service type Sushi is the Yakitori House, and inside of the Land in Future World at the Sunshine Season's Grab-n-Go area, you will find some.
Also, at the Swan is Kimono's Sushi Bar.
The Polynesian at MK has a sushi bar. Not sure if it's part of Ohana's or not. My bf went there and said the sushi was amazing.
It's not, it's right next to Kona Cafe. It's a coffee bar in the am and then sushi bar. I had the California Luau Roll - fresh jumbo lump crab, pineapple, and avocado, during my stay at the Poly and it was so tasty!