Where do I find the encryption codes for backups?

The actual encryption keys are not kept anywhere in any human readable or user accessible format. For iCloud backups, encryption is handled entirely between the Secure Enclave hardware and firmware on your device and Apple’s secure iCloud servers.

For iTunes encrypted backups, you set the password, but it is never available in any recoverable form somewhere other than your keychain or your password manager, assuming you saved it to such a place.

If this is an iTunes backup, see if anything here helps.

Backups - encrypted backups in iTunes.     

The encrypted password is in the backup, you wrote it down and put somewhere or in your head.

Note: You can still backup but to restore from the backup you need the password.

If you are using a Mac and had iCloud Keychain enabled, try that.