You don't. At least not in the US. They show is on BBC 1, I believe. You can find it on Bit Torrent though.
I should make one correction. You can see it sort of on the Discovery Channel. The show is still called Top Gear but is watered down. They have to remove much of the music and the stage shots are redone. This is bad since the show is unscripted. For the US version, they reinact what they said. They do a lap on their test track and that part is all redone also. (has to do with the music.) Overall, it's waterdown and not nearly as good as the original. That doesn't mean it's bad. I think it's still better then the US shows.
If you want to get into bittorrents, I can help you out.