Hello @kucougars85
This is a great question!
You could check some of inbox messages under the Sent folder if you have not already to see if any of them still exist there for the specific course. Another option would be to act as another student in the past course and see if you can locate the thread of inbox messages that were sent out there.
I know that the admin tools area allows you to look at Notifications that were sent to individual users. This can sometimes help if the inbox messages were deleted by the instructor or student. It helps to know the date and time that they were sent out when using this feature.
Sometimes Canvas support can look at any notifications that were sent out, which would include inbox messages. They usually need to know an exact date and time for that also.
Hopefully this helps!
-Colton