Do "air ferries" for civilian vehicles still exist?
There are a number of suitable airplanes - C-130 Hercules, C-17 Globemaster, C-5 Galaxy, AN-124, up to the rather monstrous An-225 Mriya. Cargo versions of airliners don't usually handle vehicles - they are designed for palletized cargo while the above cargo planes have ground-level ramps.

If you want to fly with your car somewhere it's just a matter of calling someone who owns one of these airplanes and putting down your no-limit credit card. 

As you surmised, the reasons they don't operate scheduled service is financial - boats are much cheaper, and not enough people want to go where a boat is impractical.