Here is the service manual for your Thinkpad. (The Thinkpad S3 is the same as a P40 Yoga, a Yoga 14 and a Thinkpad Yoga 460)
Scroll to p.131 to view the necessary pre-requisites and then the procedure to remove the button I/O card
Here is a link to a supplier of the Power button sub board (part # 00UP122) for your Thinkpad. It is not a recommendation to use them, it is just shown to give an idea of the price. There may be others that suit you better. Just search online using the part number only to get results.
Here is a link to the parts catalogue for your Thinkpad. I cannot find an actual power button actuator (if this is what you mean) listed in the catalogue. Perhaps you may have better luck in finding it.
Hopefully this is of some help