What cable/adapter should I buy to connect a MacBook Pro's Thunderbolt port to a TV via HDMI?
Dr. Bott have HDMI adapters that can transmit audio as well as video, assuming your MacBook supports it (yours does): http://www.drbott.net/product/5499-MDHD/

Then, get a Thunderbolt to HDMI adapter by Apple which transmits both video and audio. Note that Thunderbolt is backwards compatible with Mini DisplayPort as far as these cables/adapters are concerned.