In some ways the axe is easier to throw than even a good long and heavy knife. When you throw an axe, you usually bring your arm up partway, the upper arm maybe parallel with the ground, or not even that. Then you just drop your arm real fast, and the axe takes care of the rest. The momentum of the head is considerably greater than any knife. You don't have to throw hard to have dramatic effect.
Targets also suffer.
Taste plays an important part in selection. If you're interested in the Hibben, you find it in the Atlanta Cutlery catalog , and probably in a few other references at that link. Otherwise, here are a few other pointers to axe sources Starfire Swords has got to have one of the better initial offerings in this area. Here you will find something a little different in the Townsend catalog . The Sunshine Knife Outlet has lots of good throwing hatchets and small axes.