You can buy several varieties of hydroxyapatite toothpaste on Amazon now. Before 2021 or so it was a bit of a pain - a couple Japanese and Singaporean brands with spotty availability.

Edit: as alluded to by sibling comment, there are confusing claims about the exact size and structure of the hydroxyapatite ingredient used. Several brands claim that their particles (?) are smaller than those used by Novamin or by other brands and therefore more effective.

The concept of remineralizing the surface of existing teeth is totally different than what this article is talking about, though.