Try your local Salvation Army store. Near me, they have a lot of old computers (think 486)which aren't much good except for parts.
I like the solar idea, you could probably run a CPU fan off the guts of one of those solar walkway lights.
or here I just found this for $66
Built-in solar cell and DC fan make the Solar Vent an easy solution to remove heat and odour in RVs,boats,garden sheds and outhouses.Installs in a 12 cm (4.75") diameter hole and comes on automatically,exhausting air upwards. Helps reduce condensation in boats too!
The Solar Vent moves air at 32 cfm's. Dimensions 17 x 9.5 x 17.5 CM (6.75" x 3.75" x 7")
http://www.advancedenergyonline.com/catalog/application...&%20Solar%20Fans.htm