Power and volume buttons flex cable replacement

Yeah, sorry, I'm afraid asking the iFixit Store guys to check in the back isn't going to work; if a part isn't shown on the website then they don't have it. Since iFixit only sells genuine Google parts for the Pixels, they can only sell the parts that Google sells and as far as I can tell no one is yet selling the cable you're looking for, and that includes several other sites that also sell genuine Google parts. I have to conclude that Google hasn't made that part available yet.

Unfortunately, your phone is new enough that the aftermarket industry hasn't caught up with making that part for it either. So at this point all I can suggest is to wait it out and keep your eyes open for the part becoming available, or consider sending it in to uBreakiFix, Google's official repair provider; they may or may not have access to parts the rest of us can't get.

I hesitate to post this as I have no information on the reliability of this site, but it appears there's a site in India that lists the part you're looking for. I suggest checking them out thoroughly, but as far as I can tell, they're the only ones selling what you want.

Power Button Flex Cable for Google Pixel 9 Pro XL - On Off Flex / PCB by Maxbhi.com

Let us know what you find out; your research could very well help the next person that comes along with the same issue.