More Info:
These buttons are called "Mushroom Head" or "Mushroomhead" switches.  You want the momentary type, which are a little less common than the twist or pull type emergency stops.  If the blue one on that panel has a big spring under it, it's possible that it was a retrofit of push / pull to turn it into momentary.  If you get a regular momentary, you shouldn't need to worry about the spring.
But again, they run about $30 each, so you might just be ahead of the game by buying the panel.
RandyT