there are all types i wood saggest one with a built in siren driver like a car siren,it depends, you could just buy one from e-bay or just hit yard sales,simple is better like one that is 120v AC with built in siren driver no need for a transformer or external driver board. most are 12v with built in driver. but one that is 120v ac and built in driver means you only have to mount-it plug it in and your ready to go.
if you see some transformers at yard sells or thift stores that are suitible & cheep than pick um up and take advantage
of the car sirens there cheep at yard sells & e-bay about 10 bucks. good sirens from alarm stores about 40 to 50 bucks. you could just buy a driver board and run the horns, i try to take advantage of what i can find cheep and make it work. good used stuff is good in my book. money is tight right now.
thift stores are good i picked up 2 transformers 1 was 1.2 amp & the other was 1.5 amps for 1 dollar each. some car sirens to for 2 dollars each. & i got one for my key-pad that white one is 9v 200ma for a dollar.
here was a good scrounge my neighbor got mad one day and smashed his lap top and through it off his deck into the woods a couple days later after he moved i went and got it ,and got a 14.5 volt transformer a 250 gig 2.5 sata harddrive and 4 gigs of memory and it all works,the hard drive is in a external incloser usb and i use it on my lap top. thank you !!
about 120db's is about the loudest you can find for reasonable money.
i went with the bells becouse they where 20 to 25 dollars brand new & 120v ac & 110 db's, & i didn't have to scrounge transformers. when you need them you can't find them,and when you don't need them there all over the place.