Some places to try:
http://www.allelectronics.com/make-a...C.-Fans/1.html
http://www.bgmicro.com/index.asp?Pag...S&Category=100
http://www.crazypc.com/products/fans/fansmain.htm
The first two links are more or less liquidation sites, so the selection changes almost daily.
The last link doesn't have 5v fans, nor any as small as 30mm, so may require modding.