Reef's in Indian rock ( across from the Lobster pot) have cut fresh tuna that I used for sushi was great. Call the owner to coordinate . I have also bought Norweigan salmon froze it for safe eating, then defrosted it in the refrig over night.
If you are willing to cross the bridge, Star Fish and A. W. Bell fish in the village of Cortez have sushi fresh seasonals. Star has unfrozen fresh tuna consistently. In season, Walter Bell has the best prices in the state for stone crab claws.