I had the same problem you had finding the rear bumper... so what i did was order the coupe/sedan bumper. cut the bottom halves of the stock and the rieger bumpers off and molded the rieger bottom to the stock top half. this also makes it easier for mounting because it will mount like the stock rear bumper
I know its a lot of work but it turned out perfect. I will post pics as soon as its painted and on the car.
Oh and I just bought one of the replica Infinity rear bumpers which was a lot less money than the authentic. I couldnt see myself cutting up a $450-500 bumper haha!