You can also buy them on eBay, unofficially. Of course, the police unions have gone after sellers there, for some reason.
In one state the union went so far as issuing registration-like tags for your FOP bumper sticker, so that if a cop was pulling you over, he'd know whether you were "paid up", so to speak. Of course, it's only about support, and not legalized proactive bribery, just to be clear...