I remember those. I never understood them myself. If I want it to be packed tighter I used a smaller stuff sack. (In fact I never did use theirs.) I didn't see the point in the extra weight and complexity.
Maybe you could have a tailor add one to an over-sized sack you already own. It would be easy to do.
The new Thermarest sleeping bags are coming with that double cord system, but on the storage sacks, not the stuff sacks. Again, I don't get it. I don't want it compressed more for storage.