Frys is the place to get these things. Large selection, low prices. Some fans come with a small packet of rubber nubbins for each corner. These fans vibrate on sheet metal, and rubber nubbins silence fans.
For increased silence, use 9 volt or 6 volt power supply. These are 12-volt fans, but the expected 12 volts run them vigorously enough to add audible noise.
Better yet, with a wall wart that puts out a half ampere(400 - 500 milliamps), you can soldier two fans to run in parallel.
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