They're native to Ahch-To, the planet where he's been in exile since the events of Return Of The Jedi thirty years ago
The island of Skellig Michael in Ireland is covered in them (puffins, not Porgs), and is being used as a location for Episode VIII
Episode VIII director Rian Johnson confirmed this. Also, a baby one is a "porglet"!
Is that... is that a feather sticking out of Chewbacca's mouth in this still from a behind-the-scenes video? Guess the Wookiee's not the only thing that's chewy...