What other animals can one substitute for ivory?

Unicorns possibly if you and find one.

Nothing else really grows teeth that big.

Maybe just don’t use ivory at all and don’t worry about replacing it? Functionally, synthetics and ceramics can do whatever ivory can.

I think fossilized walrus teeth / tusks can be used in place of ivory.

Bone is a common substitute.

@Blackwater_Park bone i too porous.

@Tropical_Willie Bone is still used as a substitute in many applications that previously used ivory. There is also simulated Ivory and synthetic ivory, but it’s somewhat expensive.

From the OPs question in 2016 comes this answer from @dabbler: I wonder why their name doesn’t show in red?

“I have read of genetically engineered / artificially grown ivory and it’s pretty new.
The idea behind it is that since it’s real at a cellular level folks will buy it in lieu of the real thing stolen off the face of a big murdered animal. I think it could work if they can get the costs down.”

Also, antlers and horns from deer, moose, etc.

Plastic polar bears.