So far I’ve gotten lucky with acquiring used ones. You might try posting a WTB in the forum. I guess I’m in the market for another as well, as somehow one of our users handed the fob back to me with no guts inside
How do you use the Arrival Sensor? If in the car, and you have a spare USB socket, even better is the Advanced Arrival Sensor by @iharyadi, It’s also Zigbee, has better range and vibration sensor for the same price.