AS far as practice materials, a lot of people use various plastics. I have seen scrim done on most of the fake and faux ivorys like Micarta. White hard plastics of just about any kind should work for practice but the polyester plastics seem to be favored as an ivory substitute. I have seen people practice on antler, horn and bone. You have to watch out for porosity on those materials though.
Many of the knife material suppliers like Jantz, Texas Knifemakers Supply, Masecraft, etc . Should have stuff to practice on.
As far as suppliers of real ivory here is what I turned up with a google search.
http://www.boonetrading.com/
http://www.elephantivorytusks.com/
http://74.220.215.229/~kowakivo/