Googling "large push button switches" yielded this:

[Link: delcity.net]

I first thought explosion-proof switches would be the place to check, but most of those have a guard around the button designed to keep it from accidentally being pushed -- you have to push right in the middle. This is an example of that:



All Electronics (not far from me) is a GREAT source for stuff like this.

There's this, from All:



And this from Allied. Yes, the button is plastic, but it will probably last just fine:



As mentioned, access companies should have something that will work. The plate that you smack to open a hospital door comes to mind!