Where do you get sashimi grade fish?

Right don't laugh! I have major urge for sashimi nothing exotic just like salmon and tuna really : ) ......but I can't just pick some raw fish up at Sainsburys from the deli counter and slice it up can I? I read somewhere that sashimi grade fish has to be frozen for a certain period of time to kill the germs and it's best to buy farmed salmon (for raw consumption) because they don't have ring worm or something. Help much appreciated!