If you want the chain guard, probably the manufacturer would be the best place. They're expensive though!!
There was a TBNer who made his own. Cut chain into short lengths (height of the cutter's front opening) and welded them to a piece of flat (might have been angle) stock. The assembly was bolted on to his cutter. Labor intensive, but not so expensive. It's been several years since I'd read this..
Another option is to purchase a 6" wide strip of some kind of hard rubber (or you could use old truck mudflaps). This is bolted to a angle stock which, in turn, is bolted to the cutter. My old Land Pride cutter has those rubber guards (from the factory).