I got an email and picture from a web visitor (Paul) who built a trebuchet from a kit he purchased. He also sent the link to where he purchased it from and it looks a lot like a treb that you can also buy through various places including amazon.com. I have included what he has to say about building from this kit and I have included the link to where he bought it and where you can buy what looks like the same one on amazon.com

Here is a picture of Pauls trebuchet:

And here is what he has to say about making it:

It assembled very well, but I think they're optimistic when they say that a 9-year-old could do it.  (A couple of bits taxed a 51-year-old!)  I can confirm that it works much better with a 3/16 brass rod as the main crossbar - I got 9m (about 30') with a small stone yesterday.  (The original wood bar was showing the strain!)  Two pics attached, which you're welcome to put on your site.  The nuts weigh about 300g (11oz?) each and I found them on old UK railway tracks - they used them to hold the rails together in the days before welding.  (So no, you won't hear on the news that the 07:50 to Orpington has de-railed!)
No vids I'm afraid - my camera only does 12fps ... :-(
The larger project is now on the drawing board - basically a 2.5x scaled up version of the Pathfinders kit, which should just fit into my car.  (They're small in the UK!)  The support structure will be simplified, and with a 'teaching' angle: variable arm velocity ratios from 3 to 5, and variable sling lengths that can be clipped in and out using dog clips.  Anything to get the new generation interested in making things, doing stuff, then applying science to understanding how it works.  
Shall update!
All the best,
Paul.

Here is where he bought it:
http://www.pathfindersdesignandtechnology.com/kits_trebuchet.htm

And here is the same one on amazon.com:

Pathfinders Medieval Trebuchet Wooden Kit

Just add glue, scissors and an appreciation for the physics and art behind these medieval siege engines. High-quality models hurl a small ball more than 15 feet in 1 throw, with adjustable pads that change the trajectory angle and encourage experimentation for greater accuracy. Pre-cut, pre-drilled solid wood pieces are easy to assemble; models can be build in about 1 to 2 hours.