I think Marflex would sell parts directly. It's worth a call. Say you had a camera with scales in meters and preferred scales in feet or vice versa. I don't think they would make you send the camera for such a straightforward replacement. I've asked to purchase spare parts when sending my camera for repair, and Martin has supplied them.
Somewhat by luck I have found a scale for a 135mm, though Burke & James and not Linhof. I am going to try that first, and see how well that goes. Depending upon that experiment, I might order a scale for my 180mm, or maybe a different one for the 135mm.