Can anyone recommend a good hard-stop repeatable focus ring for the HVX200?

Zacuto makes one, as does Chrosziel, I think Schneider/Century does, and others.
The best one I've seen is custom made by Clairmont Camera, but they don't sell them as far as I know-- they only come with their rental HVX's.

Are you looking for a follow focus unit or something that mounts on the camera itself and doesn't need rails?

Are you looking for a follow focus unit or something that mounts on the camera itself and doesn't need rails?
The latter.