Although this isn't really boxscores, hockeydb(dot)com is a great site that has individual player stats from the 70s-Present. Pretty much every 'pro' league is represented, in addition to Major Junior.
The other suggestion I have would be the microfilm archives of your local library. In Windsor, the Windsor Star is archived (in it's various incarnations) on microfilm going back to the 1800s. These microfilm archives can be found at both the University of Windsor's Leddy Library, and several branches of the Windsor Public Library.
I'm sure most other OHL cities would have a similar setup at their respective library system, although 'searching' with the convenience of the Internet would be impossible, you'd need to know the dates you are looking for.