You can buy a capo at most any music store, they come in all shapes and styles and don't cost much for all they do. If you just want to try one out on your guitar and are feeling inventive, you can make your own capo using a pencil and rubber bands!
If you want to learn more about a capo, here's an extensive article on Wikipedia.
Thanks for visiting my website, please be sure to watch me competing in "The Voice of Poland VI", thank you! -- Paloma