Not cheap, but the silence and performance of Noctua fans is legendary to PC enthusiasts. In this case, their tan/brown color scheme works rather well.
I use a 120mm Noctua NF-F12 fan, a generic 120mm metal fan grill, a Noctua NA-FC1 fan speed controller, and a DC wall adapter for 4-pin fans. This is plugged into the switched outlet of my Yammie CR-1020, so it turns on when the amp does.
The power supply board on this amp is known for getting hot and even desoldering some components and scorching the top metal vent over time. Since I put the fan above, as an exhaust, the metal vent barely gets warm. And I can't hear it at all. Hope you find a setup that works for you. Cheers.