Thanks for reaching out. While the back plate for the Nest doorbell (wired) isn't sold separately, we'd like to check for other options. When and where did you purchase your Nest Doorbell (wired)? Also, are you in the same country where you purchased it?
You can also reach out directly to our Google Support team for assistance.
We understand where you’re coming from. However, we won’t be able to process a replacement for your accessory since this is already out of warranty. You can reach our Google Nest Support team for further assistance.
We’ll be here if there’s anything else.