hiya sheriff, i don't know about seattle, but i buy sushii grade tuna, salmon, octopus, and yellowtail in an asian (technically korean) market near my house. they have a sushi section that offers whole pieces of the fish for making sushi at home. octopus legs are really cool to freak people out at home too.
as far as seared tuna, ahi is great, but you could do it with other types of tuna as well. i like to buy yellowfin that is frozen and cryopacked right on the boat. rolled in toasted sesame and pan seared, it is deelish served with a ginger/soy/sesame oil dipping sauce.