At this time, replacement treads are only available for the predecessors of model 11120, the "two-in-one" with swing-away steps.
This style of chair represents one of the earliest stepstool chairs made by Cosco; dating back to the 1940s, it continues to be a popular item today. In the late 1950s, more treaded stepstools debuted, including early versions of stepstool chairs with a fold-up seat. These had a larger tread size than the "two-in-ones." Unfortunately, tooling for current versions is incompatible with the originals.
If you are looking to refinish a model with larger treads, we apologize that no options with the Cosco logo are available. There are many online groups that are dedicated to refurbishing vintage Cosco items, and these would be a good resource for suitable substitutes, as well as how-to tips. Tread for current models is 2.5mm thick, and should you choose to restore with a generic rubber tread, these are available online through Amazon and many other retailers. Tread sizes for current models can be found here.