Sashimi or Sushi Grade is a lose term, with no legal meaning; and hence it's meaning is more "aspirational" than real.
It means top grade.
I don't know what the percentages are but a huge amount of fish sold in the BEST sushi-ya was frozen on the boat. For instance, the odds of anyone on this forum eating fresh fish from Japanese waters outside of Japan are very low.
In fact, freezing is considered the best way of making sure the fish are parasite free when sold.
If you want to do a good job of cutting and serving raw fish, but don't know enough about fish to tell by eye whether or not it's "sashimi grade," that's the place to begin.
Start by learning to buy whole, wild, salt-water, fresh fish; by looking look at the eyes, the gills, the condition of the skin, and smelling it. Learn to "fabrciate" it for cooking. When you can cut a whole fish into glass smooth fillets -- stop and take a look at them. That's sashimi grade.
There are supers which do carry "sushi grade." In my neck of the woods they aren't at all uncommon, but are rare to the point of non-existence through most of North America. Super, specialty fish market, or something in between: Look for a fish monger who has his fish on ice -- not wrapped in covered trays in an open refrigerator gondola -- and who will let you examine the fish before you buy.
It's no different for buying larger fish which the fish monger fabricated. If it's wild fish, looks good, feels good and smells good, it is almost certainly good; even if it was frozen.
Farmed and fresh water fish present other issues. Some you want to avoid, and I don't want to get into it.
Bottom Line: Know your fish, know your fish monger. Don't serve raw fish until you do. If you don't have sources for top grade, wild, saltwater fish in your area forget about making sashimi altogether.
Hope this helps,