Its a bit more complicated than that.
The Centurion and Barbarian used to be regular features on the global map, then they discontinued them, and people complained about it,as you could only then get their materials from Uber invaders,or by purchasing coffers,for clan points in the store.(you still can )
I do not know when the clan store coffers where first added.
Fairly recently,both of these two "extinct" invaders made a return to the global map, however they do not remain as long as the rest of them do, and apart from one occasion thus far, only appear for 24 hrs.
It looks like you can get the equipment from the Avarin invader.