well it was one post 4 years ago.

but at the end of the day most people with a 3d printer could design and make one inside a few hours.

Looking at the picture it's just a flexible 'stick' that attaches to the chairs normal joystick thread.

Given that ninjaflex was the first flexible filament and it had only just been released at the time: It was more a 'look what can be done with ninjaflex' article then anything else.

These days there are any number of flexible tpus and pla's.
So find a local 3d printer and get them to make you something :-)
If you're in the uk - let me know.