I did this on my CMC on a 75 VW IRs chasis. I don't recall where I ordered these but I had to cut them which is no problem as the one end just spiral screws onto the tube.
I used a piece of PVC pipe inserted into the body heat tube and placed the flex tube over it. The pictures show the white PVC and the tube clamped over it as well as the heater box.
Hope this helps.
Michael