It is usually a good idea to build your own machine (you learn more about designing it), but often a well made kit can provide some inspiration. Many places sell trebuchet kits, but the best kits generally come from Ron Toms's Catapult Kits (One of his kits is pictured on the right). He also has a nice section specifically for trebuchet kits. These kits have a educational focus, covering some physics and history.
Below are some sponsored links that may potentially go to sites that sell catapult or other kits: