Where to purchase sushi-grade ahi tuna in Arizona? (or purchase online overnight shipping)

"Sushi grade" means that it is safe to prepare and eat raw. In order to do that, it must be frozen to kill any parasites. That means it either has to be:

• Frozen at -20° C (-4° F) for 7 days; or
• Frozen at -35° C (-31° F - "flash frozen") for 15 hours.

There aren't any official regulations about the fish itself or its quality, and most sushi/sashimi distributors have much more stringent rules of their own beyond the freezing guarantee.

With fish, you have two safety concerns: parasites and bacteria.

Freezing gets rid of parasites. It does not kill bacteria. You need heat to kill bacteria, that's why officially, food is only considered safe after being cooked to a specific temperature. Eating thawed uncooked fish is officially unsafe, and if you tried to sell it to people, the FDA would come after you.

This being said, if your personal safety standards are not as high as the FDA's (which are extremely conservative), nobody will stop you from eating it. It shouldn't be more dangerous than sushi, as long as you eat it immediately after thawing. There is the small probability of it being kept around some time before freezing (as opposed to sushi-intended fish which should be frozen on the boat immediately), but as fish deteriorates really quickly after death, you should be able to notice it smelling fishy if this is the case.