I received my plugs from Amazon. A visual inspection (per the numerous videos online) made them appear to be genuine. Then I did the resistance test as specified in this
video and they passed (values were 5.22, 5.46, 5.94 and 6.24). And finally, I reached out to Niterra North America. I got a very quick response (under an hour) requesting some close-up photos of the plug as a whole, the tip and the stamped code. I sent those right away and got confirmation that the plugs are indeed legit. So my final cost at Amazon with tax was $35.23. But the Niterra guy did caution against buying from non-authorized sellers such as Amazon. YMMV. So, you can get a bargain price and checking for fakes is pretty easy. If they are fake, just return them.