Generally just go buy one, any one.
A lot of people like the Panaflow fans, they are known for being high quality and quiet.
I say just save your money and order a cheap one, I personally spent $2.95 (USD), on my last 6 and all have been running at 4000 RPM, for roughly a year, with no problems.
My Desktop: <A HREF="http://Mr5oh.tripod.com/pc.html" target="_new">http://Mr5oh.tripod.com/pc.html</A>
Overclocking Results: <A HREF="http://Mr5oh.tripod.com/pc2.html" target="_new">http://Mr5oh.tripod.com/pc2.html</A>