Third party controllers (even officially licensed versions) are nearly impossible to get replacement parts for unless they happen to use the same parts as the official first party controller.
The silicone conductive pads would generally not be available to buy for this controller and they are not the standard ones from an official xbox controller.
Have you lost it or is it not working / damaged?
I believe your only options would be to get another of these controllers used from ebay if it is worth repairing or possibly use a different silicone conductive pad from another controller, but the chances of finding the right size is very slim.
If you would like to send over the measurements we can have a look at see if any of our controller silicone contact button pads are the right size, contact me here or on twitter @zedlabz