I've tried in many ways to get hold of sirens from big cities. It's a vicious circle, as nobody wants to stick their neck out and actually give permission. The best bet would be to get in good standing with the foreman of the City or County public works department, as they'd be the ones who would do the job of removing it if/when the politicians tell them to. Unfortunately, it's nearly impossible to get a city or county to give you the time of day. I agree that nobody should "just go take it down" cold turkey, but on the other hand, that's probably the only way it's going to go unless you have a strong tie with the local government.
I happen to know that two of the sirens in the Fort Mac Arthur museum in San Pedro, CA, were "just taken down off the poles" at night. But that doesn't mean I'm advising it, as I'm sure it could be written up as a felony against you if caught.
Most of the people here who do somehow manage to get sirens from public domain, have made friends with the fire chief, or the town manager, etc., in smaller towns and cities. That way, you might be able to get someplace with your efforts. One of the Chrysler sirens from Los Angeles County was put up and sold on eBay several years ago, and one of the 500T's was taken home by a County worker who is interested in sirens.
Charles