I can only speak for Los Angeles County Sheriff's communications and LA County Fire, but they are not in the public domain, and you WILL need permission to"legally" (hint hint) reuse them in film, video, or audio recordings. If you reuse live recordings, just scrub them of any identifying information, including unit numbers. But it's the easiest thing to recreate your own. Just make it up. If you're my age (60's) , you remember "One Adam Twelve, see the man, 459 suspect there now." Or if you're REALLY my age...
"2150 to headquarters"
"Headquarters by" (from "Highway Patrol" starring Broderick Crawford, 1955-1959)
Get a $20 Radio Shack scanner, set it for your local departments, and just listen in. Unless you have a trunked monitor (monitors two separate frequencies simultaneously, not merely scans) you'll either hear dispatched calls OR officer calls... but they're not transmitted on the same frequency. Go for monitoring the dispatched calls. BTW, SEB/SWAT /EOD are usually encrypted. And the phonetic alphabet used by most law enforcement IS NOT the same one used in aviation or by the military, so don't use "Alpha, Bravo, Charlie..."