I think the Linear Paper Ivory Micarta is one of the closest looking substitutes because it has "grain" to it and real ivory has grain. Many of the polymers don't have that grain layers.
There is a material that knife makers suppliers sell called alternative ivory. It looks very close but is also very soft, not durable at all.
The linear paper Micarta also scrims great yet it is durable.
The inlay on this knife is ivory paper Micarta, I made it for my wife at Christmas a few years ago.
This one was a custom order