It's only natural to want to share some culture with your soulmate; this allows for a type of deep-rooted connection that can be hard to attain through mere conversation and intimacy. Sharing culture with a date is pretty easy if you're asking out a person who's been your next door neighbor since childhood, but as someone living in a country which, on the whole, goes about things much differently than that of your ancestry, it can be a bit more difficult. Luckily for you though the United States is a big mosaic which grabs influences from all over the world, so sifting through and finding the perfect Japanese American relationship is definitely possible.
Japan has a huge influence over American pop culture courtesy of being the king of certain markets such as console video gaming and animated television shows and movies aimed at adults rather than children (America totes some adult comedy cartoons like The Simpsons and Family Guy but Japan completely dominates the more dramatic adult cartoon market with anime powerhouses like Death Note and Bleach). Going to conventions devoted to these types of things (as well as broader spectrum sci-fi conventions and such) will allow you to meet plenty of Americans who love your culture, as well as many Japanese Americans who are proud of the mark that Japan has been able to make on American entertainment. Putting together a particularly good costume is all you have to do to easily spark conversations (you will be approached) in these types of environments. Obviously the topics of discussion will most often center around the focal point of the convention itself, so be up to date (reading online reviews of shows and games doesn't take much time and can give you the basic knowledge necessary to pretend that you're a total elitist guru even if you're just a casual fan).
As far as population by region is concerned, the west coast is your best bet when it comes to the prospect of meeting other Japanese singles by chance in ordinary situations. Major Californian cities such as San Diego, San Francisco, San Jose, Los Angeles (home of the well known Little Tokyo), and Anaheim have fairly high densities of Japanese Americans compared to most cities. In the northeast, Boston and New York City have a pretty high percentage of Japanese individuals as well. If you're planning a vacation, pick Hawaii, where a whopping quarter of the population is of Japanese decent. It's never necessary to be in a Japanese dominated region to meet Japanese people of course (the United States has every race everywhere), but if you've got options on where you can be, you might as well shoot for the places where targeting a date will take less aim. For a map detailing Japanese population by region visit http://upload.wikimedia.org/wikipedia/en/4/44/Pctjapanese.png.
If you are a Japanese woman, then you are already highly sought after by a huge percentage of American men. It's hard to say exactly why this is, but the exotic beauty really has a tendency to make the homegrown guys go nuts. There's no shame of course in choosing to date people who share your ethnicity, but don't discount the American men as in many ways you already have an edge over American girls in regards to attracting them.
As far as language goes, you need to be sharp on your Japanese and your English to have the best chances of being able to communicate most fluently with other Japanese Americans. If you were born in America or moved to America during your youth, you've probably picked up English as your main language somewhere along the way; if you're of the more newcomer variety then this might not be the case. Either way, it's in your best interest to make the effort to fully learn the language that you don't speak as well. It's not even uncommon for people born in America but raised by their Japanese parents to speak a hybrid of the two languages when speaking to someone who they expect to know both languages. So take the classes if you need to (the internet offers plenty of resources here; type “learn English” or “learn Japanese” into a search engine and you'll be looking at a monstrous list of sites that want to teach you for free) and give yourself the highly desirable trait of being bilingual.
Style is pretty big in Japanese culture (easily more so than in American culture, where “underachiever who just woke up” is one of the hippest looks out there), so make yourself suave. As a Japanese man looking to draw affectionate stares from the prettiest girl in the room, your best bet is to look as though you either just came from your corporate job (suit and tie) or you're on your way to a dinner party (button down long sleeve shirt, slacks/khakis and dress shoes). Jeans and a tee shirt have their place for everyone, but dressing to impress has a way of adhering to the Japanese dating culture. Women have less to think about here as guys have a tendency to like you if you're attractive, but you'll still probably want to look well groomed and be wearing new clothes that aren't from Walmart.
The Japanese have been immigrating to America steadily since the late 1800s (with about 5-10 thousand coming in every year at this point), and with racism dying down and acceptance flying forth from all corners of the country, there are really no shortage of places in the United States to find the perfect single individual. So long as you have a nice grasp on your own culture, as well as an understanding and appreciation for what the United States has to offer, you're bound to find yourself united with a Japanese cutie in no time.