Like Kevin said, you cannot just walk into a shop and have a plate made. While the styles of plates available differs somewhat from state to state, all states require registration prior to the issuance of license plates.
Many states, like here in Wisconsin, will issue what's called Temporary Plates which are good for a short period of time. These are most often used right after a person purchases the vehicle and is waiting for their permanent plates.
When a plate is lost or damaged, the owner must apply for a replacement plate and this can take a month or more.
Many of the states still make their plates in their State prisons.
Using a plastic "show" plate or an immitation plate while driving on the highway is VERY illegal.
Your best bet is to call California DOT offices (or the DOT office of the state where you're first unloading the vehicle) and discuss the situation with them.
Basically ANY operation of a motor vehicle on the highway without proper registration displayed is a violation. Your own country's registration is O.K. but normally to have a vehicle actually registered in a state requires the owner to be a resident of that state. But like I said earlier, you might be able to get a temporary intransit plate.
Hope some of this makes sense.