Check at your local hardware or building supply store. Some do carry replacement wood handles. Also check the farm supply stores.
Of course, you will then most likely have to ... reshape ... them a bit to fit your hatchet. And don't be afraid to look at full-sized axe handles to cut down to fit. I once used a full-sized axe handle for a hatchet by shaping/fitting the butt end to fit the hatchet eye (the other end was broken off). A little re-shaping, and it made a nice handle. And I left it long at first, but eventually cut it a little shorter (but still longer than the original hatchet handle).
The other option is to get a board or tree branch and "carve" your own. If you get a branch with a little curve in it, they can sometimes work out pretty well.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands