How would I properly cite this source?

I am writing a historical, non-fiction book. Two of the individuals of importance in my book both graduated from the same medical school in the early 1800's. The school happens to still exist today. I knew they attended but didn't know when they graduated. So I sent an email to the library at the Medical School and they replied to my email and listed each man's graduation date.

So since this was communication between myself and the school, how should I probably cite my source for the information on the individuals graduation dates?

I did that , most websites say that personal communication is not recoverable data so should not be included in a references page. From what i have read, it should be included in text on the actually manuscript. But this is recoverable data, all someone has to do is email the library like I did so I am not really sure.

Chicago, although I will be publishing with a University press so they may want to change the format.

I did that as well. They said that they have that information entered into a computer and can just do a search...the search is not accessible to the public. The woman who I am dealing with thinks that they might have a book in the library with the information in it. Se is supposed to be getting back to me but nothing yet.

Thanks, I appreciate it. Very useful link as well.