I'll be a little more tactufl then the previous poster.
Welcome to NICO.
Just a few things though. Be sure to read a few of the stickeys here in the Skyline forum, as it seems that the Department of Homeland Security is cracking down on illegally imported Skylines. So, unless your car is a MotorEx car with bond relase papers, you have about a 99% chance that your prospective Skyline is an illegal import. So, read up on the stickeys to be aware of the risks you could be getting yourself into.
As far as parts...depending on what it is you need could vary how hard it will be to get parts.
Used engine, suspension and drivetrain parts might be a little easier to come by, either using NICO's classifieds, ebay or other classified postings on the internet.
New engine parts might be a little more difficult. Some companies are now selling new parts, such as cams, pistons, etc. on their websites. I believe that
http://www.enjukuracing.com carries a few RB parts, so you might want to check them out.
Body pannels, interior bits and the like could prove to be futile there in the states. And, unless you know someone in Japan that has a contact at one of the dealers, you're gonna have a bit of trouble.
Again, welcome to NICO and enjoy the ride.