Uwajimaya, hands down. Or if you want to go up the supply chain a little further, head down Rainier to Mutual Fish.
I know that Uwajimaya has several types of sashimi-grade tuna in at the moment ranging from hot-pink (CO treated) frozen blocks of imported whatever to some pretty decent bluefin tuna at ~$41/lb.
Don't forget the fresh wasabi, btw. It's in the produce section - one of the very few places in the US where mortals can reliably buy fresh wasabi at retail.