Radio Shack sells 12v PC fans that are cheap and exactly what you need. they are about 4" square usually, perfect for attaching to carbon tubes.
if you are going to be growing that many plants though, will a small filter like that handle the smell? might want to get something a little larger. i have seen people buy a piece of Gutter Guard and roll it into 2 cylinders, just like the pencil cup idea. trick is, you can make the filter any size that way.
best of luck!