While I’ve posted recipes for several  different  kinds of  sushi  on this site, I have never published a recipe for making nigiri zushi, the kind of sushi most people think of is the sushi, in spite of several requests to do so. There are a couple reasons for this, which you may want to consider before embarking on your own nigiri sushi making experiments. One reason, as I’ve written about before , is that it’s quite difficult to get the nigiri part (the forming of the rice ball and placing of the neta or topping) right. Of course you can practice this, or use a sushi former, or even - if you get fanatical about it - a sushi robot. But the more serious reason is that raw fish is something to be very, very wary of at all times.
I worked at a sushi restaurant for a few months, and my mother ran the same restaurant for many years. So I’ve been around fish when it’s being prepared. It takes a lot of skill, training and experience to verify that fish is fresh enough and good enough to serve as sushi. Quite a few times I have seen the itamae-san at the restaurant I worked in reject fish that was not up to par - and this was supposedly top quality fish that had been supplied to them by one of the handful of fish wholesalers in New York, a sushi mecca outside of Japan if there ever was one.
How can you be sure that the raw fish you are getting is safe to eat? Buying from a reputatable source is essential, and you must buy sushi or sashimi-grade fish only. Something to keep in mind though is that the best sushi-grade fish is snapped up by the restaurants. In Japan, there’s such a demand for raw fish that the supply is reasonably good, and I would tentatively say that the same holds true for large cities outside of Japan with many sushi-savvy consumers.
Here in Zürich, the only raw fish I eat is tuna, which is sold as sushi-grade. I stay away from all other raw fish - this is a landlocked country with not much of a tradition of preparing fresh sea fish. (Freshwater fish in general are not fit for sushi.) In New York, I occasionally got some sashimi from a Japanese grocery store, or sushi-grade fish from Citarella, but I didn’t go beyond that. I don’t have the strongest stomach to begin with and there’s nothing worse than the hours of agony when you get even a mild case of food poisoning.
Another problem with an amateur preparing nigiri zushi is that they tend to handle it way too long. Hands are warm, and warm raw fish is not a good thing. Notice how your favorite sushi chef’s hands work very rapidly and lightly (if he’s a good one, of course).
So, if you do want to try making your own nigiri zushi, or using raw fish in another manner such as as sashimi or in hand rolls, always proceed with caution. For myself, I prefer to have nigiri at a good sushi restaurant.
(I should also point out the common misconception that sushi means raw fish. Sushi in fact refers to the flavored rice. Any dish made with sushi rice is therefore sushi, and sushi does not have to use raw fish, or any fish at all.)