I think this is going to be a do it yourself affair.
Get two pieces of sheet steel four inches wide by twelve inches high by 1/8 inch thick. (Or half inch plywood) Fasten these upright and flat on the wall and attached to the studs (not centered) so as to achieve support 68 inches apart. Use at least four screws at least 2-1/2 inches long per plate, two near the top of the plate and two near the bottom. Have holes or other treatment of the plates to receive the hangers of the mirror.
As I type this reply I see an ad on this website for supporting systems with horizontal metal bars. Each would have to be at least 20 inches long to span two studs behind the mirror and extend out a little to provide the support 68 inch apart. At least two screws have to go into the stud nearest each mirror hanger to fasten the bar to the wall.