There's a circuit to scratchbuild in one of the electronic books published by Babani - might have been one of the 555 IC titles. Can't help with an off the shelf and my circuit books all went down the charity shop.
Were they not steam powered? I have a vague recollection of when I was in the Navy, getting hold of one to fit to a more modern ship and we had to modify it to work with compressed air.
As you can see by the responses electronics can so easily emulate any sound you want that air or steam powered horns are almost redundant.
The sound can be created by soft ware, hard wiring or by digitally recording what you want. I have just thrown away a children's book with a sound generator producing 8 different animal sounds. So look for an electronic kit that produces a variety of sounds. It is common now days for burglar alarms to have a range of sounds by switching so you can identify your own alarm. god luck olewill