Can store bought salmon be used for sashimi?
Not being aware of your location, some general tips:


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.
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.
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.