For the old bouts pre-1980, the old Ring Record Book used to have a section on purses for heavyweight title fights going back to the time of John L. Sullivan. Some major metropolitan libraries may have the old record books on their shelves or archives.
I am not aware of any current source that lists all the purses.
For specific fights, if you have access to any internet newspaper sources - such as newspaperarchives.com - the news accounts of the fights and the pre-fight activities frequently list the purses.