Maybe the Japanese Market on the 79th street causway
I buy alot of fish at Casablanca fish market on the river but Ive never bought tuna there. You should call ahead to see if they have tuna.
Also, I find the tuna at Fresh Market in the Grove to have excellent quality tuna as does Epicure (but epicure is quite expensive)
18295 Biscayne Blvd, North Miami Beach, FL 33160