I just saw an ad in one of the magazines that featured graffitied cars. Don't remember the maker though. I do remember that I wasn't overly impressed.
Athearn is now making "pre-weathered" cars, but it just looks like a quick mist of gray from the spray gun. I'm not impressed at all.
Another option is graffiti decals. Or, you can paint them on yourself. Sometimes doing this type of work on your models can be very gratifying.
Or, there are folks who specialize in weathering cars and putting graffiti on them. The cost can be reasonable or outrageous, as these guys usually put there stuff on E-Bay and it goes to the highest bidder. Now, concerning these guys, some of their weathering work is absolutely astounding.