Most Bonsai enthusiasts started their hobby with a ready-made Bonsai tree, bought in a Bonsai shop or received as a present. Although you will not experience the fun of creating your own tree, it is a great way to get started and get a feeling for the basic care for Bonsai trees. Once you get enthusiastic about Bonsai you can eventually learn growing them yourself.
Most large garden centers sell low-priced Bonsai trees, although these are usually not of high quality. Many specialized (online) Bonsai shops sell bonsai trees as well, though at slightly higher prices. Check this article for a store near you; Bonsai trees for sale.
Some basic advice for buying Bonsai trees:
Go to the basic bonsai growing techniques forum.
Learn more about indoor Bonsai tree care and outdoor bonsai care.
Bonsai Shop (Netherlands): Bonsai Shop and Der Bonsai Shop (Germany): Bonsai kaufen.
Read about selecting quality Bonsai books.