You went into 3 stores and were told they couldn't get your plugs. You went into NAPA and were told NGK didn't make them. It's obvious that for whatever reason, NGK removed that application from these vendors' catalogs. The fact you had one in your possession only means to the clerk that NGK made them at one time, not that it is a current production part. If you went into a Z specialist shop and were given the kind of information some of you have received, the dissatisfaction is understandable. To expect a guy at a parts counter who knows nothing more about your car than what his computer tells him is being unreasonable.
GatorJ 1996 300ZX NA 1970 Fairlady Z 2006 Ford Escape Hybrid 4WD 1973 VW Super Beetle Sports Bug