How do I find the keychain login password

The passwords for the users login keychain and 'local items' keychain are normally the same as the user computer login password and normally kept in sync.

If they get out of sync and one possible way is when manually moving a user account/home directory between two computers then this might be one possible cause.

Another is if the file/folder permissions become incorrect, for example if the file/folder becomes owned by the wrong user account or either the POSIX or ACL permissions get set wrong.

Sometimes the Mac operating system will have an issue and merely rebooting will 'un-confuse' it.

Another possibility which does not sound like it would apply here is if the relevant keychain becomes damaged. I would expect a different sort of message and for it to create a new empty undamaged replacement if this was the case.

If the file is undamaged, has the right ownership and permissions and it is merely the password is incorrect and cannot be guessed then likely your only solution will be to delete the existing keychain(s) and let it create new fresh empty ones on the next login. As you probably have already tried, you should try both what you believe to be the current correct password and any previously used passwords.