I just got a catalog from New Haven moving equipment today. They have more locations than just New Haven, they are in multiple large metropolitan areas. They stock moving equipment for movers, but they also rent equipment. While some of their equipment is beyond the need of most theatrical endeavors, they do carry (and reasonably priced) j-bars - or lift levers as they call them. They have two styles - one with rubber wheels and one with steel, in three different sizes each. They even have a reference section in their catalog that discusses the rigging capacity of their webbing.
All in all its a site worth a look: