McDermott uses something they call Recon Ivory (Urethane) - But I think this is a proprietary material. I tried searching for it and its nowhere to be found
I like the way it looks on my G501
Here is a web page that sells an Ivory Alternative... see if it might be worthy.
If by substitute you mean something that is white I would say go with Mother of pearl or Holly wood. As far as inlays go there is any number of things from MOP or Abalone and any number of exotic woods such as Pink Ivory wood and stones. Ivory is in my opinion one of the least interesting materials for inlay. People will debate it's value as a ferrule material and playing qualities but it is a total bore as an inlay material. As far as I am concerned it will not be missed at all in the making of pool cues.