As per FreedomGundam there are collectors who keep the boxes and may be willing to let go of them.... If they haven't thrown them out=
The majority of kept boxes are from the vintage G1 era, but you never know... I keep some my modern era boxes...
Sites like Cybertron.ca is a great place to search. Just start up a Wanted Thread of what you're looking for.
I know from experience there are always a few members out there with something you're lookin for... No matter how odd or random=