You can also purchase a pack of O-rings—search "retainer rings" on this website.
Bobbin rods for Tensioned Lazy Kates used to have cylindrical knobs, instead of spherical ones. The rods are interchangeable.
How to Install
Insert the bobbin rod through one rim of the Tensioned Lazy Kate, then through a bobbin, then through the other rim of the Kate. Secure with a retainer ring.