I think Apex or Avon uses them on the front of the tubes (kind of a combo grab handle/cleat kind of deal.) OK, I checked; I was thinking Apex, but both have them.
Might try hitting local (or whatever you can find) dealers and see about getting some spares ordered. I was unable to come up with a source in a brief search.
Apparently, Henshaws and PolyMarine also make some, though I don't know about distributors on this side of the pond.
I seem to recall seeing them as accessory parts somewhere, though, so don't give up too easily.