wow, Now that I read this I realize what those minature fans lying around my junk pile could be usefull for, so I don't know if you can buy them anywhere anymore, but I do have two fans that would be about that height, and are rather fat (thick) too, I think 3 would fit nicely, all I can suggest is search around secondhand places, maybe old (OLD OLD OLD) laptop parts, etc.
It's pretty simple actually, just use the concept of a centrifugal blower.. Use an 80mm fan (it fits almost perfectly if you lie it flat, airflow pointing downward. Then cute out one side of the fan (this is the side you're going to have facing out of the 3 1/2" drive bay). now simply cover the top of the fan (the side that usually intakes the air) so that air is forced in through the side you cut open, and then downwards. If you'd like, I can draw it out for you, just ask.
Ahhh! It's 2003 and wait, whats this!? My computer didn't blow up! ... Well damn, there goes my new Athlon @ 333FSB and my inssurance claim... heheheh