Well, you have to buy one that is already in California, since of 200(something) it's illegal to import Skylines. You also can't bring skylines from other states in because California has different car laws.
Also, what you can do is, buy a skyline from Canada, and drive it back down to California and buy a savaged title Nissan S13/14 and register it in California. Rip out the firewall of the S13/14 and the frame that has the VIN number on and cut out the Skyline's frame and weld on the S13/14's frame that has it's VIN. So basically, you have a Skyline with a S13/14 VIN number on it, if cops ask, tell them that you have a S13/14 with a crazy skyline body kit on it and you have the VIN number to show it. I know a couple people in the bay area who have done this.
At least there's hope for your dream car. My dream car is the Audi S1, which is impossible to obtain since there's about 10 of them in the world and it is strictly illegal to drive in America, and also, it's known as the world's dangerous car....well it's among the group of deadliest cars ever made, Group B, which is banned worldwide.