Thank you for the links. For those that are interested psX controllers will work. The buttons map as such:
I'm still not sure where to actually connect the buttons to the controller though. The buttons on the bottom row are spaced 5.5" apart from their centers. The top row buttons are spaced 5.5" apart from the bottom row buttons and between the bottom row buttons (more exact measurements would be nice). As far as buttons go, these are what I have found that can easily be purchased individually:
Illuminated Pushbuttons (Jumbo Round)
I have heard that they are loud and cannot compete with other types of buttons on the more difficult songs though.