circle c and b&d both work, b&d is cheaper, circle c is much better quality in tube thickness, machined replaceable ends, not welded on stub shafts.   the circle c picture above looks like a generation 2 which is what I use.  I bought a generation 3 set when they first came out probably ten years ago to replace mine when it wore out or became damaged but have not used it yet.  Im beginning to think I better change it before I wear out or I might not get to use it.  the Generation 3 has a slightly smaller tube diameter and slightly thicker rubber but the same overall diameter.