Several different styles of switches have been used and they have different faceplate. The ones that Dale has and the ones shown in the GE catalog (links above) use a rocker style switch.
Mine use a lever switch and take a different coverplate. You might have to replace switch also to get a matching coverplate.
But here are a couple of options to look at. If these where used in a number of tract homes look for full line hardware stores in the area. They often will have specialty items used in those homes.
Try some older electrical supply houses. But take an old coverplate.
If they are like the ones that I have they appear to use a Sierra Modular system. This was a bracket that would take upto 3 devices. Sierra had switches, pilot lights, and receptacles (ungrounded) that would fit in there and you would get 1,2 or 3 hole coverplates.
I don't have one to compare with, but from memory they are the same and the mounting brack and coverplate on mine say Sierra.
I can't tell for sure, but they appear to be the same as these.