I have come to realize the hard way that Arduino is a boutique product, not a commodity. Board production seems to be done in small batches. With leading-edge technology sometimes some small part must have gone unavailable and Arduino does not react very quickly.
Fortunately all is open source so you are free to use a site like Osh Park to get a board and then solder on all the parts. Sometimes some small company has done this already and is selling the assembled product on Ebay, caveat emptor, or else AdaFruit or SparkFun will have a reliable substitute.
This does make it difficult for people who write books and teach courses because last year's notes have gone obsolete. I wish that the Arduino team would pay attention to the distribution chain but they always seem to be chasing the next technology. Welcome to the bleeding edge.