Device for toggling power to another device via raspberry pi GPIO?
The simplest device you can use will probably be a solid state relay.

Example1, Example2

Most of these will be grossly oversized for your siren, but they're cheap and they solve the problem with only one part. You could use a regular relay, except that the coil draws more power than the pi can supply. An SSR will give you the same effect, but will draw far less current from the pi.

A smaller and cheaper way to drive your siren would be to use a mosfet as a switch. This is how it would be done in a commercial product.







simulate this circuit &ndash; Schematic created using CircuitLab

The circuit is simple, but you'll have to carefully choose the mosfet to make sure it will be fully turned on by the raspberry pi's 3.3v. Most common mosfets will not. If you go this route, a mosfet you could try is the IRL2203NPBF.