Had the same problem. Since I live in a place where Casio doesn't have a service center, I decided to design and print my own replacements on my 3D printer. I was able to print a batch of 10 in about an hour and they worked great. I plan to replace all the original ones with mine to prevent future trouble. You will need to use a flexible filament to print. I used a Ninjaflex clone and that worked great. I plan to replace all the rubber ones with my printed version so it shouldn't degrade over time.
Here's the link for the design if anyone is interested.
https://www.thingiverse.com/thing:4886427
Enjoy!
