I also forward the Yate Loon suggestion for inexpensive quality fans they are quite, push a fair amount of air, undervolt nicely, come in such a lovely orange color, and in black. But unless you want to exclusively run it at 12v, 7v or 5v you will need to buy a fan speed controller. I have read Yate Loons sometimes do not react well to PWM type control but Rheostat type fan control works well.
Also you could remove your rear fan grill and replace it with wire grill. And by remove I mean take a dermal, tin snips, or wire cutters to it and replace it with a wire fan grill. Basically the less restrictions the airflow has the better off you are.
Actually its strange but I kinda like the orange color of my Yate Loon fan.