Do you know where I could buy one?
Does your case have a location for mounting one ? Take a look at your user manual. Not a lot of end user cases support such a switch. I have only seen it properly mounted on brand name boxes.
If your case doesn't have a location for mounting one, then you won't be able to buy one to fit to your case, unless you want to experiment.
You can use something like this:http://www.ebay.com/itm/DELL-OPTIPLEX-DIMENSION-CHASSIS-INTRUSION-SWITCH-UC635-/390080992837
But you will need to figure out a way to mount it, with Loctite or similar glue.