Siren sound effects are available on GRSites.com and AudioBlocks.com as of 2015. GRSites.com offers a wide selection of siren sound effects, including fire truck siren, tornado warning siren and coast guard boat siren. They are free and are available for downloading in WAV and MP3 formats.
AudioBlocks.com offers royalty-free siren sound effects, such as long siren, short siren, fire truck siren, firehouse siren and ambulance siren. You can preview these sound effects on the website for free, but you need to buy a subscription to download these files. As of 2015, AudioBlocks.com charges about $99 a year for unlimited downloads.