Steve,
Have you looked at Renaissance? I'm fairly sure that I recall seeing something there - could have been P/E part, though.
Tamiya morgan 4/4 has rubber straps but considerably different to those you are looking for.
I also seem to recall seeing a diecast with quite similar straps and remember thinking at the time that they could be "stolen" - if I manage to remember what it was, I'll let you know
Old age....phew!
HTH