I have never seen a switchplate like that but I had the same issue in my old house. I used a double switch plate and installed a switch on the "dummy" side that did nothing.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

http://www.smarthome.com/_/Dimmers_Lighting_Appliance_Control/Build_Your_Own/Switch_Trim_Plates_Covers/Trim_Plates_Accessories/_/z/2PN/1Pv/1Qj/nav.aspx

This place has close to what you want, they have "build your own" snap together plates.

__________________
'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor