First, the term “onyx” has been misused. The material in question is
dyed quartzite and always has been. Onyx is a mineralogical term for
a form of calcite, which would be totally unusable for what we use
"black onyx" for.
Having said that, the best current source for black onyx is probably
just buying larger pieces from a reliable source and re-cutting as
needed. Plenty is available from the usual suppliers. I’ve seen black
tourmaline (schorl) used as well as black nephrite jade, but I don’t
think anyone usually cares as long as the material is black.
Personally, I’d welcome a durable, evenly-colored substitute. (Not
synthetic, that’s misusing another term!!!)