Hey, Cathy.
Unfortunately, I don't know where you would be able to get one of those little plastic buttons, perhaps you could find somewhere to get one 3d printed. Fortunately, that little plastic piece is just there to make it easier to press the small momentary switch that is the actual button. In a pinch, you can depress the button with a pen.
You can find the button by looking at the space where the missing plastic piece was. if you look inside the bottom hole (directly below the screw) you should see a little white dot.