Livin,
I would troll the library, you have a photo of what the finished product needs to look like.
Measure the track width, and the diameter of the ball that it fits on. You will need a socket, so you want it fairly tight.
Look at the library and pick a young millennial that is coming out and see if they want to help you. If you had an old worn out one it would be easier to visualize.
I would even ask the librarian who teaches the class on how to use and let them show you how to do it yourself...
If they are that hard to find, you could probably sell them on the samba for some overpriced amount...
good luck!