if you can find one of the 9C1 Malibu's from the late 70's and early 80's, you might be able to find a push bar that will fit. those are the only G body police cars that i know of.
or maybe a late 70's 9C1 Nova might fit, as well.
no offense to anyone here- but coming to a GN board and asking where to get the parts to do something truly unique to your car is kind of a waste of time. every year, i go to the Car Craft Nats and look at a lineup of 20 identical GN's with identical mods- well, some of them have different wheels- but the cars were built identical, so they are kept more or less identical. so, go all "Mad Max" on your car- don't forget to put a "turbo" button on the shifter- and make sure to park it right next to all the other turbo regals at every car how you take it to. i bet yours gets more looks, and people spend more time looking at it. and i bet you enjoy your car more, too.