Wake up early and head out to your local fishmonger. Get them fresh so make sure you wake up at the butt crack of dawn to get there. Restaurants usually get the first pick but if you get there early enough you can get some really fresh seafood for really good deals.
Last year I was fishing off the coast of Costa Rica and we caught a tuna. The boat captain filleted it right there and that had the be the freshest sashimi I have ever eaten.
No soy sauce or wasabi was present though.
