Can store bought salmon be used for sashimi?

Not being aware of your location, some general tips:

1. Buy your fish from a fishmonger, and tell him/her what you are using it for. You want to do business with somebody whose business is selling fish and only selling fish. They are going to know what's been stored to eaten raw standards in a way that the just above minimum wage fish guy at your grocery store doesn't.

2. Don't buy it early. Buy it the day you mean to consume it or the day that you are going to prepare it. This question can help walk you through that.

3. Take a cooler to get your fish. You have to maintain the freshness, that means not allowing it raise in temperature anymore than you have to.

I wouldn't trust anything from a grocery-store fish counter to be fit for raw consumption.
http://www.sushifaq.com/sushiotaku/2008/01/31/where-to-buy-sushi-grade-fish/ has a lot of info, and suggests http://www.catalinaop.com/ . I've never bought anything from them, but it looks like they're probably a good source if you want to buy online. If you're in the SF Bay Area, there are a number of places to buy it. I usually go to Tokyo Fish Market on San Pablo in Berkeley.

Living far away from the ocean for much of my life- fishmongers and freshly caught fish are hard to come by.

Luckily that doesn't matter that much. The freshest fish are the ones that are frozen on the boat they are caught on.

Suburban grocery store fish counters can sell you frozen fish in small quantities. I buy tuna or salmon steaks in .5 pound portions. They make excellent and very fresh tasting sushi when thawed.

It will keep indefinitely when solidly frozen but as soon as you thaw it the normal rules apply- use it that day or it won't be fresh anymore.

So don't order the fresh or the fresher fish. At Finney's, if you're wise, you'll say, "Fetch me the finest, French-fried freshest fish that Finney fries!" (Sorry, I couldn't help it- I've been reading to the kids.)

I have used smoked salmon to make sushi rolls with success, I would not try it with nigiri as the difference would be extremely noticable. Depends on how authentic you want to be.

Definitely frozen fish would work better for sushi, but industrial and processed, don't try to freeze it at home or you're at risk of infections. I'd recommend going straight to seafood wholesalers, as they have already the fish processed for sushi to sell straight to restaurants. I had to make takoyaki and octopus sushi for an octopus festival in Cali and contacted http://www.discefa.com/en/ to sell it chopped and ready to make sushi with it. In case you don't need that much quantity, you can ask the wholesalers where they sell it to and contact those guys. Hope this helps!