What an epic show.
Yesterday I discussed the theme from Stargate Atlantis, and I mentioned something about Joel Goldsmith not being the one who composed the main title theme. Although that remains a fact, information regarding the title theme song is somewhat confusing and all over the place. But, I did my best, and to sum up as simply as possible, below is the final verdict.
The Stargate SG-1 theme song was arranged by Joel Goldsmith. It is a compilation of the various scores composer David Arnold did for the original Stargate movie. And, as for the end title theme, that is a definite Joel Goldsmith original. There.
David Arnold is an English composer who scored five James Bond films and Independence Day (film) among others.
An official soundtrack is definitely available. A search will lead you to various versions and releases, but those are essentially one and the same. The soundtrack includes the opening and closing themes—first and last songs respectively.
As with Atlantis, all opening title versions are available in YouTube. However, below, from season 9, is the only embeddable video (jerks). After follows the end credits theme taken from the soundtrack.
End credits theme.