In Harold Urey's patent #2,690,379, Google Patents says that the patent examiner added a citation, but the patent itself doesn't say that. If I want to read the patent examiner's notes (assuming it's more than one sentence), where could I order a copy?
The citation google patents listed is from the patent. It appears on the last page of the patent.
The image file wrapper of any patent is available to the public. It is the closest thing to "examiner's notes" there is. It includes all of the back-and-forth written communication between the USPTO and the applicant. For applications filed from the middle of 2003 onward the file wrappers on on-line on USPTP Public PAIR. For application before that they need to be ordered. Many commercial entities offer this service. It usually charged by the page and file wrappers can be very long.
One service is REEDFAX.