just email them and ask for Port Multiplier Backplanes (CFI-B53PM 5 Port Backplane (SiI3726)). They will set you up with a sales rep.
And they are super helpful.
"I am currently looking for just the backplanes themselves but can't really find any for sale. Anyone have any suggestions?"
Hey, about the Port Multiplier back-planes. CFI-Group makes them and sells them. But you have to call them in Hong Kong and they are only open during their day hours...