Todai! Great Japanese seafood buffet right on Camino Del Rio South. They have sushi as well. I believe it's like $20 a person but their lunch prices are $15/person as I recall.
Also try O-Nami in the Mission Valley shopping center. Same idea as Todai, but less expensive.
There's also a place called "Sushi Itto" (I think), in Hillcrest on Washington Street.