If your Ampera doesn't include a cabin air filter (the Volt doesn't come with it either) the housing is already there, and you just need to purchase one and install it. it is behind the glovebox, and can be accessed by removing the thin rubber mat from the glovebox and popping the fasteners behind it to remove the cover.
The charcoal impregnated filters are available off ebay for about $15 American. (pretty good deal, since they are closer to $50 from GM)
The charcoal impregnated filters are available off ebay for about $15 American. (pretty good deal, since they are closer to $50 from GM)