Well, Nissan might be able to get you some for an absolutely outrageous price: its worth asking just to hear the price
Given these are an OEM item, and the R34 wasn't sold here, the best option is obviously to buy where they are most plentiful (i.e. Japan). They come up all the time on Yahoo Japan auctions, and there are a number of auction agents that can help you get them here.
LW.