Pepboys sells a few kinds for about 50 bucks. Its got a pretty cheap speaker for wherever you mount it, but for an extra 20 dollars you can get a really nice speaker from Radioshack. I use the cheap one, but my friend spent the extra 20 and its real loud. That's what I would do. If I were you.
The sirens are similiar especially to people who don't know the difference in sirens. Its got tons of animal noises, and ice cream truck jingles.
As someone pointed out, with the real sirens YOU WILL GET ARRESTED. Plus it's way more fun to chase someone down with an old Adam-12 red spinning light and ice cream truck music.