Where can I buy a 13/16 unicycle seat post
Are you sure, it’s 13/16"? All Seatposts (unicycle as well as bike) i’ve ever known are 7/8" or bigger. What unicycle do you have?
It’s a1969 Montgomery Ward 24" unicycle.
One option is to do something like this: (click on the 26)
There seem to be some vintage 13/16" seatposts for bmx. They are 7/8" at their plain top and would work with the Qu-Ax adapter.
Example for seatpost: Chromoly 13/16" bicycle seat post with 7/8" seat mounting - 400mm - Porkchop BMX