Try Comair Rotron, Inc.'s "Whisper" fan product. I have a #027117 that runs on regular 115VAC. It is sealed ball bearing has 1600 RPM, 55 CFM of air movement, and an acoustic noise level of <30 dba. Several months ago I took it to radio shack and ran an A/B comparison of all their fans and the Rotron was the quietest by far. I checked out the quietpc.com web site and they do advertise for some quiet 12VDC fans, e.g. ~1700 RPM, for 20 -40 CFM with 20 dba. If that is enough air movement the price is right at quietpc. My rotron cost about $27.00.
Make sure to mount the fan on something that will isolate the vibration of the housing. I tried mounting the fan on a foam-rubber mouse pad and using rubber washers around the screws to mount it to my cabinet. The quietest, now don't laugh to loud, was to set the fan on top of a hockey puck and then place another on top of the unit. The isolation this provides results in a surprisingly quieter fan. Okay you can stop laughing now. Probably a baggie filled with sand or lead shot would do the same thing but the only thing I had laying around were the hockey pucks.