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Although I don't see why it would be illegal... Police badges aren't.
That is not a generally true statement. Badges, police indicia, ID cards, etc. are all "regulated" and based on use, intention, sourcing and jurisdiction there are various laws (with criminal penalties) that control them. Just because you can buy, say, an NYPD badge online does not necessarily make it legal to possess unless you are in a particular jurisdiction and stay within specified parameters.
Depending upon which country / state you are in, the laws vary.
Even bonafide, real Hollywood studios working on actual, big budget productions have been ordered to control their use of realistic police / LEO props. The FBI internal newsletter (the "Informant") had an entire article on them cracking down on shows. They cited such well known productions as X-Files, Mancuso FBI, the FBI, Greatest American Hero, Silence of the Lambs, etc. More recently, there was a TV series featuring Ernie Hudson as an LASD training officer with a rookie. They wore real LASD patches and the dept. got a cease and desist order on them. If you saw the show or have it on DVD, you can see the floating overlay on all scenes showing the patch.
The Tommy Lee Jones movies abut the US Marshals (Fugitive, USM) used, IMHO, the most realistic prop marshal ID cards. It could be that the movie had the support, approval and a tech advisor from the USMS. The TV series, "The Marshal" also had a pretty close ID set.
The USMS recently (within 5 or so years) modified their IDs a little, but the old double card set is still commonly seen.
Hope this helps a little.