The soundtrack of "The Keep" was by Tangerine Dream.
The reason you cannot find it is that it has never been released in its entirety, due to legal problems.
Quote from Amazon.com:
"Performed by the new age gurus TANGERINE DREAM, the soundtrack of THE KEEP has never been released in its entirety. In 1996, members of the group re-recorded two pieces used in the film in new remixes, and in 1999 Tangerine Dream released a special "official" release which contained only two pieces from the film compiled with unused studio test sessions (a bootleg circulating in the early '90s contained the same material). To date, the actual music used in THE KEEP has not been released due to legal problems unlikely to be resolved.
While the video is periodically released for sale, fans of both THE KEEP and TANGERINE DREAM await the possible arrival of a special edition of the movie on DVD, which it is hoped will include an isolated music score and additional footage, especially the rarely seen second ending shown occasionally on television."