What switch do I use so as to be open when pressed, and closed otherwise?
You want a Normally Closed push button - the switch contacts will be connected unless the button is pressed.  A single pole, double throw button may be easier to find - that would give you both Normally Closed and Normally Open contacts.

A common name for a switch that is operated by things rather than directly by people is MicroSwitch (TM Honeywell, I think).  Microswitches are available with a wide variety of actuating levers.