You can get it at dental supply houses that specialize in dental stone and things like that. I have a company in Oregon picked out because of price. It's actually surprisingly affordable. It's about $25 for 50 lbs, but that doesn't include shipping. The expensive part is getting set up with a mold. I'm looking at about $150 for the mold after I get my first prototype shape ready.
You ask if I "have any pictures of one." I'm assuming you mean cane handle. Well, no, which is why I asked here. I want to create a prototype cane handle and be able to repeat it again and again. Silicon mold technology and dental stone can used to manufacture bone-like handles with very exacting tolerances.
"Can it get cavities?" Oh yes, but with a good sealer on it, I doubt it. I know I mentioned tea-staining, and with a clear coat that would be great. Yesterday however, I researched siliconizing the handle in order to give it a non-slip surface. This would make it easier to lean the stick against a wall without slipping, make the handle more non-slip for hand, and give the handle a layer of shock absorbing silicon to protect it from chipping (which I think is possible, but unlikely). But then the teas-stain idea goes away, so I might as well use something else, like fiberglass or carbon fiber.
Finally, you asked about where I had heard of it. Well, I'm a contractor, and I've spent a fair amount of time researching the various kinds of plasters available in the building trades, and through that research I learned about dental stone and mold making. The stone is incredibly hard, like teeth without the enamel. It might even be stronger than that.
Technically, cured plasters are metals, and the harder versions can actually actually ring when you strike them, like a tuning fork. Dental stone is the hardest kind of plaster there is.