Wireless thermometers are cheap, last years on a single set of batteries, and are made to be exposed to the elements. Because of this, they are a good alternative to building your own wireless temperature sensor. The following document describes the wireless signal and protocol for one type of these thermometer types.
This is a non exhaustive list of devices for which the following specifications should work. You can recognize the outdoor temperature sensor by its distinguished shape.
Example brands for this thermometer: