My understanding is that the spare tire can only be purchased as part of the wheel/tire assembly. Occasionally you will find one for sale on eBay or on NSXprime (often by an owner who has installed a big brake kit and no longer can use it).
The spare is not designed to be driven on for anything other than emergency purposes. If you need it for more than just getting home, you're probably better off picking up a set of used wheels and using one. Or, borrow a wheel/tire from another local NSX owner who has aftermarket wheels/tires and still has the stock ones in his garage.