Silo 3D (www.nevercenter.com) is a good inexpensive subdivision modeler that more or less pioneered retopology tools. You can give their demo a try and see of that works for you.

Another app more or less dedicated to the task - TopoGun (www.topogun.com) is another good app - they're currently in beta (and for some reason snatched up all their download links) but that's another app to consider for taking high density models and cleaning up their topology. (Make sure you check out their beta 1.2 video)

-Will