DVD. You see this is not the OVA, it's the tv series that was, I believe, released last year in Japan. I assume this, because on the DVD there is a special feature that has a voice actor, and the singer of the theme songs, at a event on christmas 2004.
The first DVD is available at WalMart for around $19. You could probably find it at a video store, but it would cost somewhere around $30.