It looks like habitat systems is no longer in business and they made the best cages, by far. I don't know exactly what material they used, but I have heard that it was a hollow core pvc used in the hog farming industry.
I know NERD managed to reverse engineer their cages somewhat and at least a couple other people tried. There must be a reason why all the other PVC builders use a different material, but I think it would be a fun project to try.