I got a thick string out of the material in walmart, then went over to the fishing section and got the bells people put on their fishing poles for night fishing. Tried them all together and hung it by the door. at the most I think I spent 5 bucks
I got my bell from the catalogue In the Company of Dogs. It cost 20+ dollars but it is actually a nice looking accessory and rings very loudly, almost as loud as a cowbell, so you can hear it wherever you are in the house.