www.bumperboyz.com, they are the only place that one sell me a 64 rear bumper without a core. Good prices too. Cost me less without a core than some of the places that quoted me and I had to have a core.
You can order show quality or driver quality. I chose driver quality and the rear bumper that I bought from them is better than the front bumper I sent up to Alberta Plating to be straightened and show quality plated. I might have just had good luck though.