The problem with used sensors etc is they are potentially the same age as what you're trying to replace and it is a gamble that, even if you do find a suitable unit, it's not going to be as bad as the one you already have - with the added bonus of driving to the yard, messing around removing it after walking around for half an hour finding one. I wonder when CA is going to ban smoke detectors due to their radioactive isotope content and it taking 400+ years to break down...