Most knife suppliers sell it. Jantz, Texas Knife supply, etc.
The best type I have used is called "True Ivory" and really looks like ivory. Most all are acrylics,with various powders added to get the right look.
There used to be an alternate ivory made from bone meal and acrylic. It had a nice old antique look and was not as plastic looking as the plain acrylic stuff. I don't know who sells this now ( if at all).
Another alternate for the look of an old ivory handle is the "ancient" color of ivory micarta. It is a paper micarta that looks close to ivory. The plain white ivory micarta is OK, but I really like the "ancient" stuff.