You'll probably have to replace all of them with something compatible (same dimensions and shaft size), so they all match. Sites like Digikey.com should allow you to drill down through their menus to find the available parts. Buttons may be a bit harder, this might seem off the wall but have you considered carving some out of wood then painting them, or taking one off, making a rubber mold from it and then casting new ones in polyurethane or whatever the cool kids are using for that these days?