Supposedly, and I believe it, milled coconut husk fiber is what can be safely passed if ingested. It's available at pretty much anywhere that would carry herp or herp supplies, and is very cheap. For about $4 you get a hard brick that dissolves in water to about 8 times that amount, so you can set up a couple of tanks with it. It looks almost like dirt when prepared, but a rich brown color.
I like it the best because it can hold moisture (and any excess can be, literally, squeezed out), plants grow in it, it's safe, it's clean, it's cheap, it looks natural, and it's very easy to work with. Some brands are Bed-A-Beast and Eco Earth. Good luck!