From this page, you can download SPNATI's Offline Version, containing everything in the Online Version (at spnati.net) as well as older archived opponents.
To get the Offline Version:
These bundles include extra content not needed to play the Offline Version.
These bundles need to be installed alongside at least one of the main Opponent Bundles above.
For the Event and Duplicate bundles, these opponent types are hidden by default and must be enabled in the Extra Opponents menu (wrench icon on the title screen).