Well, do you want the halter to be 'bulletproof' basicly, or is it let's say, for some fun photoshoots?
I'd definitely go ahead and sew some bells on, or hang a bell on one of the metal rings! :)
Perhaps try http://www.safehorsehalters.com/
, I know they can work Customized, so perhaps they can make it Christmas-Themed?
(Or any other halter customizing place!)