I heartily concur with Bob's Seafood. I regularly get sashimi quality salmon and tuna there, and have bought other sashimi quality fish there as well - such as swordfish, escolar, and yellowtail. The tuna is usually yellowfin. They usually have Scottish salmon which is my favorite for sushi. Tell Barbara what you want and she'll steer you right. If you have specific requests get on their lists and they'll call, and not just for sashimi fish. Once they got in a whole bluefin and I got a five pound chunk of the belly, both otoro and chutoro. Since it was 3 pm already it was all that was left; the rest had been sold to local sushi bars -- and they let me have it for the maguro price!
It's diagonally across from Nobu's, so you can have lunch while you're at it!
Some of what Whole Foods sells what they say is sashimi quality, but they are far more expensive.
I have also used the two web sources mentioned, and recommend them as well.
I make sushi all the time but don't trust the nornal markets. I have been lucky to know a wholesaler who provides fish to restaurants. My best suggestion is to go to Bob's Seafood located 170 and Olive. They provide many restuarants and at least you know the how fresh the fish is