This is where I got Izzy's bells and it has tons of info on this site. Well - look at the brightside....he knows that it's by THAT door.....I know....it's hard to hear them ringing the bell when they are that little....that happened to me with Izzy - I was in the kitchen and she must have rung them and peed in front of the patio door..not her fault...mine for not listening.
http://www.poochie-pets.net/