At this time, Celestron does not sell extra weather station transmitters.
Fortunately, Celestron weather stations transmit and receive on a frequency of 433 Mhz, which is the same as used by many other weather station brands and similar products. Transmitters or sensors are available from other companies. Most likely these will be able to be used with a Celestron weather station. Here are search results on Amazon for brands such as La Crosse, Ambient, Springfield, Oregon Scientific, and AccuRite: 33Mhz Weather Stations