Buy the fish. Japanese markets will carry high quality fish trimmed into pieces that are ready to be sliced and served sashimi-style. However, many fish markets and high-end supermarkets carry fish equally suitable for making sashimi. Nearly any kind of fish can be used—freshwater and saltwater varieties apply. Popular choices include mackerel, tuna, snapper, halibut, salmon, sea bass, eel, and scallop.
Fattier cuts of fish tend to work best for this succulent fare. Larger fish such as salmon or tuna easily achieve the nicely formed rectangles customary to the dish, but smaller fish can certainly be used as well, and many species and cuts even have distinct presentation styles.
A market that caters to sashimi makers and restaurants will distinguish between the parts of some fish because different cuts of the same fish can impart vastly different flavors and textures. For example, the high fat content in certain parts of the tuna’s belly is considered more desirable than other cuts of tuna and hence is far more extravagantly priced.
Buying fish from a reputable supplier is very important when it is intended for raw consumption (see “Health concerns” below).
Health concerns: The quality and freshness of fish for sushi and sashimi must be held to exceptionally high standards for both taste and health reasons. Unfortunately, the term “sushi-” or “sashimi-grade” is more a marketing term, less a guarantee of freshness or quality. FDA recommendations and local health department requirements for ready-to-eat foods do apply, but there are no specifics for “sashimi-grade” fish. Because there is no federally regulated grading system for fish that is consumed raw, many suppliers have set up their own parameters to measure and guarantee freshness. Knowing a little about your source can thus be helpful. Nonetheless, there are regulations concerning parasite destruction in raw fish that are enforced by FDA inspectors. This involves freezing and storing seafood at the appropriate temperatures and durations to kill parasites. It is mainly salmon that poses a parasite threat, and for that reason it has not been eaten raw in most Japanese cultures until recently. Now that freezing enables destruction of the parasites found in salmon, it is considered safe, although some connoisseurs believe it degrades the taste somewhat.