If you want to replace a broken pediment for a particular piece of furniture and cannot find it, you can look for furniture refinishers in your area.  Many skilled crafts people can research the piece of furniture and recreate the piece in their shop; and then refinish it, and repair the furniture so that no one would ever tell.   I lost a molding off the top of my 100 year old credenza, and found a person about 20 miles from my house, who could replace the piece.  You could also post on craigslist.com in your area ,about what you need, and see if you get responses.    There are many antique salvage places around too, and they buy old trim and furniture pieces and resell them.  You can look on line or in your phone directory, under Antique Salvage Companies, or Furniture Restoration businesses.  If you call a furniture restorer and they cannot help you, they will know where you should look for the pediment.  Don’t give up, the piece you need is out there.