"Sushi Grade" doesn't really exist by the way. It's not like a USDA prime stamp or anything, it just means that a fish is fresh and has been kept on ice. There are three main species of Tuna that are common in Sushi/Sashimi, the Yellowfin/Ahi, the Bigeye, and the Bluefin. These are also probably what you are going to get when you get Tuna steaks or fillets.
Canned tuna in the U.S. is always Albacore, which is not really what I'd consider to be representative of "Tuna". Especially not after it's been cooked for hours.
You will likely never know the "grade" of your tuna because retailers and restaurants won't tell you or don't know. The grades range from #1 (the most fat content/quality) to #3 (the least fat content/quality) Pretty much the only way you'll know the grade of your Tuna is to go buy it yourself at auction. Bring a lot of money.
