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 have bonsai trees for sale 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 kopen and Der Bonsai Shop (Germany): Bonsai kaufen.
Read about selecting quality Bonsai books.