For the most part, producers of commercials rent vehicles from specialty outfits that keep all kinds of things on hand, or are able to get them via private owners. Police cars, fire trucks, ambulances, municipal buses, and so on. You name it, they have them ready to film. Most every rental house usually has a matching pair of classic Mustang convertibles, a pair of some kind of classic muscle car like a Chevelle, and these days, Broncos or sometimes Scouts.
They would have no reason to have a rusty anything in their fleets, unless a producer (in that case likely a movie or TV producer) requests one for a reason. Then, they'd probably add on faux rust via a special effects artist.