[jomak]

Where can you buy a cabin air filter? Where is it located, behind the glove box?

I searched the manuals I have and found a reference to it is scheduled maintenance manual.

 

[PowerSource]

in the engine bay next to the generator inverter. Alex Pop sells cabin filters on ebay under the term "Mocrofilter"

http://www.ebay.com/itm/2012-Fisker-Karma-interior-mocrofilter-EOM-/321993065247

 

[Harleyguy]

It's located in the cowling you will see an access cover on the passenger side

 

[Fabulist]

It's located in the cowling you will see an access cover on the passenger side

In practice, how often do you replace them?

 

[jomak]

In practice, how often do you replace them?

I would guess change it when you change the air filter. That's how I always did my other cars.