Since it wasn't common to use push buttons to register credits they probably don't exist. Maybe think about using coin return buttons and housings, or simpler yet printing "25c" on some clear labels and fixing them to the buttons?
If you really want to go with the printed route I suggest getting a rubber stamp made, some ink that works on anything. Then apply your wording to the button and spray with a couple coats to fixative or clear coat. That would look keen, but it may be more attention to detail than you want to undertake. I only know this because I'm currently prototyping acrylic dice with specific artwork on each face.