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Yea eBay or several wheel companies sell them. All you need to know us the stock hub size and the bore size of the GTR wheels. They should be pretty cheap.
I know there are sites that sell them, just can't think of them right now. Some even make you the exact specs you need. The metal ones cost a little more. They are good to have, but not something you have to have. I have run plenty of wheel without them and noticed no difference at all.