I would recommend opening up the old siren and inspecting it before ordering batteries. If you like tinkering, it isn’t a big deal but you can find out if the board is corroded and damaged so you don’t end up wasting money on batteries that you can’t use. All the suggestions for spare parts are good to prevent you from being stranded and most of the other things that come up will not be an emergency so you can order parts as you need them.