Depending on the amount of work you're willing to do yourself, you could go as low as about 2mm with this capacitive touch sensor or, if you'd rather have a tactile switch use the tactile TL3315. In both cases, you'd have to solder and think about a good way to mount the button to your case.
If you want something more stock and don't need it to be as low-profile as the options above, take a look a the pre-wired button replacement kits from LianLi. Or just any pre-wired power button, really.