Try End PC Noise for a wide selection of quiet fans. I have one I got from them mounted to the back panel of my HT system cabinet running 24/7 and it is inaudible. I power the fan with a 12V wall wart. Two cable boxes in the cabinet alone generate a substantial amount of heat around the clock, and with closed cabinet doors, I need to get rid of the heat buildup.
http://www.endpcnoise.com/cgi-bin/e/std/category=Quiet_Case_Fans.html
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http://www.endpcnoise.com/cgi-bin/e/std/category=Quiet_Case_Fans.html
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