2 suggestions:
a) Make friends with a cop. I got a Mass. State Police sticker this way.
b) Scour junkyards. You'd be surprised at how many are left on windows, and if you look hard enough you'll find clear spray adhesive to put them back on with.
Now some strangeness. I have 3 MPA (Mass. Police Association) stickers on my Formula. I verified with a Sargent in the Mass. State Police about the legality of me having them on the car, because they are supposed to be for officers and immediate family only. They all have numbers on them so theoretically they can be traced back to the original officer. The Sarg checked and found out that when a car is traded in or junked, it is up to the office to remove the sticker(s). There is no law that says you cannot scrape one off and reuse it. Strange, but true! (Disclaimer: This is a Massachusetts police sticker, and verified by a Mass. State Trooper. Use discretion in YOUR state!)
Alternatively, look online for state police hats. A baseball cap laying casually in the front passenger's seat looks mighty nice.
In either case, NEVER ever ever tell the officer that you are a police officer. That will get you a night in the crowbar hotel. If they ask where you got any of the items, simply reply "I have a friend." and leave it at that. You do not have to name anyone. 
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