Jebsen and Jebsen will help you special order them if you can wait. Or, you could just order it from B&H Photo.
There a several types of 4x5 Polaroid backs. The newer ones are less prone to jam, but not as well-built compared to the older, all-metal type (very, very tough). Once you drop the newer plastic type, it will crack. I dropped mine once and I had to super-glued it. Still usable though.