Hi Vivian,
You're absolutely correct, Bushido is not a Martial Art itself, per se, however, I have personally known of a number of systems and schools of various arts that have taken the name Bushido simply as the name of their system (eg. Bushido Karate).
The term Bushido literally means "The Way/Path of the Warrior" (Bu = Martial, Shi = Person, Do = Way/Path), and is thought to have been coined in the Edo Period. In early Japanese history, the way of the warrior was known as Kyuba No Michi, the Way of Mounted Archery.
There have been a number of different interpretations of exactly what constitutes Bushido, however most agree that it is comprised of seven virtues:
Gi - Rectitude
Yu - Courage
Jin - Benevolence
Rei - Respect
Makoto/Shin - Honesty
Meiyo - Honour
Chugi - Loyalty
and, occasionally:
Ko - Filial Piety
Chi - Wisdom
Tei - Venerence for the Aged.
No matter what particular Art you study, so long as you include these aspects, you are following Bushido. But if what you are after is the Martial Art of the Japanese Samurai, then your choice becomes a little more limited. Systems such as Aikido, Judo, Kendo, Iaido, Jodo etc are modern arts, and therefore not what the ancient Samurai trained in. Karate was also not used by Samurai, in fact it was developed primarily to defend against them! This doesn't mean that these arts don't or can't include teh above virtues (and more!), it just means that the particular art was not physically in existence to be learnt by the Samurai themselves.
With that in mind, look for anyone around who teaches a koryu system, but don't expect an easy time...
Your last alternative is actually a Ninjutsu organisation. The arts taught under the banner of (authentic) Ninjutsu in the various groups (Bujinkan, Genbukan/Kokusai JuJutsu Renmei, Jinenkan, and a number of the various split-off organisations) include a number of koryu Samurai traditions, including Kukishin Ryu, Takagi Ryu (various branches of each...), and others. If Samurai is what you are after, I would probably recommend the KJJR, taught alongside the Genbukan, simply because it focuses directly upon the koryu traditions, however until 3rd Dan, you will learn an amalgm of the various Ryu-ha before you can learn an individual system by itself.
I hope this had made some sense to you, and has helped in some way.