You can order new plug or you could probably make one yourself. I had a Whelen siren that didn't have a plug so I cut the plug like the one in the pic off. Then I looked at the install manual and just extended the wires and ran them to whatever functions they were for.
That I'm not sure of. I'd probably just go to Radioshack or somewhere and buy the stuff to make my own. Someone else might be able to chime in and say for sure where you could get one online.