Depending on your set up, you may want coin buttons. If you have a coin door, this may not be an issue.
You can get the parts from Happcontrols.com, but I'd recommend getting them from some of the Happ distributors here in the Buy/Sell/Trade forum such as PonyBoy and divemaster. They both can get you better prices than Happ, and are great to deal with.
You can also go with Electric Ice buttons from Groovygamegear.com if you want the option of using RGB leds to change the colors, and an LED wiz to take care of controlling all those LEDs.