I LOVE Cargobob! It's a trolls' fantasy! you can pick up anything (cars, boats, 18 wheelers) and put it anywhere! I just spent the best 90 minutes of my life picking up people's cars! I encourage everyone to do it! And its harmless, you don't have to kill anyone. you can pick people up one the move (really hard). But usually I find someone in a store or something and nab the parked car, then deposit on roof!
You can get a cargobob (and a submarine) from the Merrywether warehouse at the southern tip of Los Santos Docks.
~For added zest; find a non-personal vehicle, season with C4 and serve immediately!