You can order PC fans and wallwarts cheap from
http://www.allelectronics.com. I go to the store in Van Nuys, so I don't know what it is like to order from them. The PC fans come in a variety of sizes.
I did notice one problem with your proposed setup. You have fans blowing both in and out. You should only have the fans going in one direction, either all in or all out. I remember reading somewhere about PC cooling (Yes I am a computer geek as well as a plant geek) that fans blowing in or out only are more efficient than in AND out. This was attributed to the turbulence created by the competing fans.
The second snag I can see you running into is volume. It sounds like you are going to have a lot of fans running. If they are all the same size and speed this could create a very loud canopy. One recommendation I will make is to either buy variable fans or install potentiometers on each of the fans to control the voltage. This will allow you to "tune" the fans by having each one at slightly different speeds. If they are all at the same speed the soundwaves will amplify. Whereas if you are able to tune them you will be able to achieve SOME cancelation and create a tone that is more pleasing. Plus potentionmeters are preaty cheap.
Hope that helps.