You can also get small amounts of Soul Cinders and Soul Ash via the mission table.
The mission levels are usually 40+, but at certain renowns in the later levels you get items that grant a ton of exp to one of your companions of choice, which basically boosts them to Level 40+. Boost the right one to 40+ even if you haven’t been doing missions at all throughout this expac, and you can do the Soul Cinder/Ash missions with the right team on any of the covenants.