I had the exact same problem/question a while back. No one from CM ever got back about the availability of those parts.

Seems the only mounting hardware available for purchase is the kit for the LGA 1700 ($5 on Amazon). https://www.amazon.com/dp/B09V3CFDP3

The metal brackets and screws included in that kit are compatible with cooler pump/block mounts for the LGA 1151 socket, but you still need the 1151 backplate.

This appears to be a compatible backplate. https://www.amazon.com/Maxmoral-Bracket-Radiator-Plastic-Backplate/dp/B08NCYP5Q9/ but I can't guarantee that the threads will match.

Other things you can try: You can make a "wanted to buy" post on r/hardwareswap or look for it on ebay.

Good luck!