WARNING: This cheat involves editing the system registry. This should only be done by those familiar with the registry as editing or deleting an incorrect item could result in system failure.

First, you must have started at least one Sandbox game. After that, use regedit or regedt32 to open the registry editor and locate the following registry value:

HKEY_CURRENT_USER > Software > Lionhead Studios Ltd > TheMovies

Change the key "Allow any Decade" from a 0 to a 1.

Now, when you start a new Sandbox game you may select any decade and will have access to most (if not all) costumes and sets.