Why don't you just fix the ones you have? Usually the only thing wrong is they get dust in the photo sensor. Remove power from the sensor (usually by unplugging your alarm and disconnecting its battery). Remove the detector cover by pushing the latch with a tiny screwdriver in a rectangular hole opposite the sounder. The cover should then hinge down. The photo sensor is inside the rectangular black plastic case (not the D-shaped case). Remove it by squeezing in diagonnaly opposite legs to release the latches. Using a damp lint free paper towel fragment, wipe off any dust in the bulb and sensor chambers. (I also had to remove a spider web). Reassemble and power everything back up. If that doesn't work, then consider buying new sensors.