I can get you a right 7/8-18, about $60 Left would be special order, but I can get you a price if interested.
Are you sure it is 7/8-18? UNF would be 7/8-14, making them cheaper and more widely available. 7/8-18 is a "special" pitch
You might be better off(cheaper) welding new inserts, with material threaded in during welding.
Try McMaster-Carr, they might have the lefts in stock