What is the song played when the Doctor and Rose are saying goodbye and where can you find it?

The song is called "Doomsday", and can be found on the first soundtrack released for the new series of Doctor Who, entitled "Doctor Who (Original Television Soundtrack)", which was released on 4th December 2006. The song is the 27th track on the album, and it is available as a single download from iTunes. There is also a second album, entitled "Doctor Who, Series 3 (Original Television Soundtrack)".