An idea:

A web connected[1] ultrasonic sensor unit, that you put[3] on the side of your car , and while driving, it recognizes[2] empty parking spaces and sends the location[4] to a cosm.com map.

[1] through your phone hotspot.

[2] since an ultrasonic sensor senses distance, it can sense if you have a car near you or an empty space.

[3] Maybe with vacuum stick units.

[4] You'll need a gsm shield for that.