From B&H Photo
B&H Photo has limited stock of the Canon ST-E3-RT wireless transmitter. This seems to be a difficult accessory to get your hands on judging by the emails I get.
This transmitter is only compatible with the Canon Speedlite 600EX-RT. There are rumors of a less expensive radio receiving flash in the near future. The last word was it would come in the first quarter of 2013.