I really like to eat salmon sashimi and nigiri. What I want to know is where do the sushi restaurants get their fish from? Do theyuse salmon steaks or filets? How easy would it be for me tofind fish with the same quality to prepare at home?
Plenty of places sell sushi-grade fish; if there’s a high quality fish shop or even a Whole Foods close to you, check it out.
If not, try Catalina Offshore. They sell sushi-grade salmon and other fish, and I’ve ordered from them plenty of times. Highly recommended.
I’m not sure what you mean by the same “quality”. There is no specific standard for sushi “grade” fish. Some of the species need to be frozen (and typically are) to kill bacteria, but that’s about it. In other words, it’s marketing speak.
Go to your local fishmonger/fish market and you’re good to go.
Again, you might want to look into how to eat some of the species raw safely (as I mentioned, it usually involves freezing).
It seems “sushi grade” fish is a marketing term.
That said one would suppose a sushi restaurant that cares about its quality would be careful in their selection of their fish.
It also seems, from below, that “raw” fish in the US has been frozen initially to kill parasites. Naturally this does not apply to stuff you catch. I guess it is just fish for sale at market.
Other countries may of course be different (a friend of mine said when he was in Japan he ate at a sushi place where a live fish was butchered then served to him moments later).
As to where they get their fish from I assume a local distributor. The restaurants you visit would probably be willing to tell you where they buy their fish from. Whether you as an individual can get it from their distributor is open to question.
Where I live, there is an Asian market chain called H-mart. They have fishmongers inside that cut sushi-grade tuna (and other fish) right off of the fish itself, and sell it right there. They package slices of the fish for sale to take out, but they also have fresh free samples and a food court that uses the fish to make into takeout-able sushi. If you’re in Chicagoland, check them out!
As far as “what is” sushi grade? Well, to me it just means the difference between a large slab of fish with bones in it, and a smaller cut of fish without bones. Obviously sushi grade fish would be the latter.
But that’s all about preparation, not the actual fish itself.
So it’s not “where do I buy sushi grade fish”, because there is no such thing, it’s more about “how do I prepare and cut and safely serve tasty sushi!”
I’ve made it from sliced smoked salmon from Kroger. 
Always make sure Salmon has been previously frozen. A few years back I scared off a bear doing his business on a logging road only to realize he ran off dragging about 10 foot of worm behind him. After that sight, all of my smoking fish or sushi fish is first filleted and then put straight in the freezer.
My easy (but not quite as nice looking) way of deboning the salmon is to use a fillet. I cut the fillet into strips about an inch and a half wide and then debone each strip using my fingers and a knife. From each strip I get some bigger pieces for sashimi and the smaller pieces for rolls or my press. But I am not a sushi chef and don’t really place any value on presentation.