I'm a cheap bastard. I just take that thin sheet of pink foam that most mobos come on, and cut/fold that to make some foam isolation-mounts. Yeah, if you're one of those people that has windows and lights and stuff, having bits of pink-colored foam all around may not be for you, but I've found that it works pretty decently to cut down on case fan (and PSU!) vibration, and at effectively zero cost. (Use a few plastic-covered wire twist-ties to secure the foam if you need to, I save the ones that come with my components. You can poke them through the foam easily if need be.)