Thanks for reaching out. While the magnetic mount for the Nest camera isn't sold separately, we'd like to check for other options. When and where did you purchase your Nest camera? Also, are you in the same country where you bought it?
You can also reach out directly to our Google Support team for assistance.
Just checking in to see if you still need assistance with this issue. We'll leave this topic open for another 24 hrs in case there are any follow up questions or comments. Please let me know if you need further assistance.