I'm a nissei, originally from California, and I also have been seeking other native Japanese people with whom to practice my Japanese. I work in Seattle and live in Lynnwood, which is just north of Seattle. I rarely see Japanese people on a casual basis. There are a lot of Korean and Chinese people more than there are Japanese. There's an International District, but no "Japan Town." It seems like you really have to make an effort in Seattle to connect with Japanese people here.
I would say it's easier to connect with Japanese people in San Francisco and San Jose. San Jose has a Matsuri Festival every year and a small Japan Town. San Francisco also has a Japan Town. The Bay Area in California has a decent-sized Japanese population. My hometown, which is smaller than San Jose, had enough Japanese that we even had a small Japanese grocery store. In Seattle, there is a big market called Uwajimaya, which is in the International District.
If you can afford the Bay Area, I would recommend it over Seattle. Seattle's a little more affordable, but the population of Japanese in the area are not obvious. Hope this helps. Good luck!