Even if there's an old fire siren laying around it can be very difficult to cough up the funds for returning it to operation. I was talking with someone at the Gloucester County VFRS not too long ago, and when I asked him if they had considered putting their old Model 5 back into service he said that they had decided they couldn't justify reconnecting it to power, restoring it, etc.
Admittedly, I'm not very proficient on the cost of purchasing new and restoring old sirens, but I would imagine the price tags would be somewhat similar. You'd have to ask an expert about that though.