The simplest and most tasteless is (really!) unflavored Metamucil. You can get powdered psyllium in bulk at the health food store, which is the same thing, but Metamucil brand seems to have a little less obnoxious texture.
Marshmallow and slippery elm are good for tender tummies and contain quite a bit of fiber, if the other is too much for them. As I mentioned before, slippery elm is becoming endangered so I'm switching over to marshmallow. As my "experiments" progress I'll let you know!