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...
I like to illustrate corruption in the US by showing them the fake badges that cops’ friends or family put on the windshield of (endemic in the NYC area) or the “I donate to the police union” stickers.
Or of course the actual police union gold cards or whatever that police spouses and kids get.