Unfortunately, the license plate you saw is unlikely to indicate anything about Starling City's true location.
Now, a "KAZ" on license plates indicates that the car is from Kansas. Does this mean that Starling City is in Kansas? Or is this just another attempt by the writers to keep the true location of the city hidden? Or, am I missing a really obvious fact about rental cars?
"KAZ" prefixes do not necessarily indicate Kansas plates, although Kansas plates issued between 1994 and 2001 might start with KAZ ("GRS 000 to approximately PAZ 999"). Such a plate would be yellow along the bottom, fading to white in the middle, with light blue along the top, with a sheaf of wheat.
Because Arrow is filmed in Vancouver, it's more likely that it was a British Columbia plate. BC plates tend to be plain white, which helpfully makes them more generic for TV (coincidental I'm sure). BC plates issued between 1985 and 2001 might start with KAZ ("LAA 000 to XXX 999").
As it turns out, Supernatural is also filmed in British Columbia, and both are under the Warner Bros. Television production company. It would not surprise me at all if the same cars, and the same license plates, ended up being used in both shows, as they're both pulling resources from the limited pool of local prop companies.
This answer from Movies.SE points out that cars driving on open roads during a shoot need to use valid local plates - in my personal subjective experience, they must do a lot of regular road shooting in Vancouver, because plates are always my first tip-off that a show is filmed there.